diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
new file mode 100644
index 0000000..e9ae4fb
--- /dev/null
+++ b/CONTRIBUTING.md
@@ -0,0 +1,136 @@
+# **How to contribute to newrelic browser exporter**
+
+#### Table of Contents
+
+[Project structure](#project-structure)
+  * [application](#application)
+  * [plugins](#plugins)
+
+[How to set up a new metric](#How-to-set-up-a-new-metric)
+
+[Tests](#tests)
+
+## **Project structure**
+
+### **application**:
+```
+─ src/
+  ├── app/
+  │   ├── routes/
+  │   │   └── metrics.js
+  │   ├── router.js
+  │   └── server.js
+  └── index.js
+```
+
+**[src/app/routes/metrics.js](https://github.com/ContaAzul/newrelic_browser_exporter/blob/master/src/app/routes/metrics.js)** - It represents the `/metrics` route, which outputs the metrics from `register.metrics()` as a string in a Prometheus like format that can be consumed by a [Prometheus Client](https://github.com/siimon/prom-client#register).
+
+**[src/app/router.js](https://github.com/ContaAzul/newrelic_browser_exporter/blob/master/src/app/router.js)** - It registers all application routes.
+
+**[src/app/server.js](https://github.com/ContaAzul/newrelic_browser_exporter/blob/master/src/app/server.js)** - Contains the code to start and configure the application.
+
+**[src/index.js](https://github.com/ContaAzul/newrelic_browser_exporter/blob/master/src/index.js)** - It starts the application.
+
+### **plugins**:
+```
+─ src/
+  └── plugins/
+      ├── newrelic_browser/
+      │   ├── javascriptErrorsPercent/
+      │   └── api-request.js
+      └── prometheus/
+          └── charts/
+              ├── default.js
+              └──  gauge.js
+```
+
+**[newrelic_browser/](https://github.com/ContaAzul/newrelic_browser_exporter/tree/master/src/plugins/newrelic_browser)** - This plugin is responsible for pulling data from New Relic's API([api-request.js](https://github.com/ContaAzul/newrelic_browser_exporter/blob/master/src/plugins/newrelic_browser/api-request.js)) and exporting it as Prometheus charts.
+
+**[prometheus](https://github.com/ContaAzul/newrelic_browser_exporter/tree/master/src/plugins/prometheus)** - Contains the metric types used to create new Prometheus charts and exports the global Prometheus `registry`. To know which metrics are supported by Prometheus, see [metric types](https://prometheus.io/docs/concepts/metric_types/).
+
+
+## **How to set up a new metric**
+
+Create a new folder inside `newrelic_browser` plugin. Inside this folder create a new file that must have a public method called `collectData` which must return a new `Promise`. The public method initializes and returns the chart:
+
+```js
+// javascriptErrorsPercent.js
+const GaugeChart = require('../../prometheus/charts/gauge');
+let JSErrorsGauge;
+
+const collectData = () => new Promise((resolve, reject) => {
+  if (!JSErrorsGauge) {
+    JSErrorsGauge = new GaugeChart({
+      name: 'newrelic_browser_javascript_errors_percent',
+      help: 'return a percentage of pageviews with javascript errors',
+    });
+  }
+
+  ...
+}
+
+module.exports = {
+  collectData,
+};
+```
+
+Please take a look at [Prometheus best practices](https://prometheus.io/docs/practices/naming/) to learn how to set up a chart's configurations properly.
+
+After configuring the chart, it is necessary to set up parameters such as `name` and `value` to retrieve specific metrics from [New Relic's API](https://rpm.newrelic.com/api/explore/applications/metric_data)
+
+```js
+// javascriptErrorsPercent.js
+const collectData = () => new Promise((resolve, reject) => {
+  ...
+
+  const names = 'EndUser/errors';
+  const values = 'error_percentage';
+
+  ...
+}
+```
+
+Now that all parameters are set up, a request to New Relic's API can be done by calling `browser.collectData()`:
+
+```js
+// javascriptErrorsPercent.js
+const browser = require('../api-request');
+
+const collectData = () => new Promise((resolve, reject) => {
+  //...
+
+  browser
+    .collectData(names, values)
+    .then(response => {
+      onSuccess(response, resolve);
+    })
+    .catch(reject);
+
+}
+//...
+```
+
+A callback method should set the metrics collected from New Relic's API into the chart. Please, see prometheus [base units](https://prometheus.io/docs/practices/naming/#base-units) to know how to set values into charts according Prometheus patterns.
+
+```js
+// javascriptErrorsPercent.js
+function onSuccess(response, resolve) {
+  const percentage = JSON.parse(response)
+    .metric_data.metrics[0]
+    .timeslices[0]
+    .values
+    .error_percentage / 100;
+
+  JSErrorsGauge.setValues(Number(percentage.toFixed(8)));
+  resolve();
+}
+//...
+
+```
+
+After setting values to the chart and resolving the `Promise`, New Relic's metrics are avaliable as Prometheus metrics in the `/metrics` endpoint.
+
+## **Tests**
+Tests are written using [jest](https://jestjs.io/). In **most cases** new test scenarios are necessary for the new developed code.
+
+Run tests using `jest` or `npm test` commands. To see coverage, run `jest --coverage`.
diff --git a/README.md b/README.md
index fc3af32..b3beeda 100644
--- a/README.md
+++ b/README.md
@@ -10,20 +10,23 @@ You will need [API_KEY](https://docs.newrelic.com/docs/apis/getting-started/intr
 ### **With docker:**
 To run:
 
-```
+```cmd
 docker run -p 9595:9595 -e "API_KEY=****" -e "APP_ID=****" caninjas/newrelic_browser_exporter
 ```
 
 ### **From source:**
 Clone this repo and go to newrelic_browser_exporter folder:
-```
+```cmd
 > git clone git@github.com:ContaAzul/newrelic_browser_exporter.git
 > cd newrelic_browser_exporter
 ```
 
 Install dependencies with ```npm install``` command and run with:
-```
+```cmd
 npm start APP_ID='****' API_KEY='****'
 ```
 
 Metrics will be exposed in ```localhost:9595/metrics```
+
+Would you like to help? See [contributing](https://github.com/ContaAzul/newrelic_browser_exporter/blob/master/CONTRIBUTING.md).
+
diff --git a/src/plugins/newrelic_browser/javascriptErrorsPercent/javascriptErrorsPercent.js b/src/plugins/newrelic_browser/javascriptErrorsPercent/javascriptErrorsPercent.js
index c410817..e9d2b85 100644
--- a/src/plugins/newrelic_browser/javascriptErrorsPercent/javascriptErrorsPercent.js
+++ b/src/plugins/newrelic_browser/javascriptErrorsPercent/javascriptErrorsPercent.js
@@ -24,6 +24,7 @@ const collectData = () => new Promise((resolve, reject) => {
 
   const names = 'EndUser/errors';
   const values = 'error_percentage';
+
   browser
     .collectData(names, values)
     .then((response) => {