Add usage to README

Closes #1

README.md

@@ -2,4 +2,53 @@
 
 > An automatic SVG converter for your CSS files, built on top of the [PostCSS] ecosystem.
 
+## Usage
+
+Right now it is only possible to use this as a [PostCSS] plugin:
+
+```js
+var postcss = require('postcss')
+var svgFallback = require('postcss-svg-fallback')
+
+var input = read(/* read some css */);
+var output = postcss()
+	.use(svgFallback({
+		basePath: '', // base path for the images found in the css
+		dest: '', // destination for the generated SVGs
+		fallbackSelector: '.no-svg', // selector that gets prefixed to selector
+	})
+	.process(input)
+	.toString();
+```
+
+Converts this:
+
+```css
+.icon {
+	background: url(images/sun-is-shining.svg) no-repeat;
+	background-size: 20px 20px; /* background-size is mandatory */
+}
+```
+
+to this:
+
+```css
+.icon {
+	/* original declarations are untouched */
+	background: url(images/sun-is-shining.svg) no-repeat;
+	background-size: 20px 20px;
+}
+
+/* same selector, but with a prefix */
+.no-svg .icon {
+	/* a png image is generated and placed in the `dest` folder,
+	 * with default settings, that's right next to the original SVG
+	 */
+	background-image: url(images/sun-is-shining-20x20.png);
+}
+```
+
+Converting is done with [PhantomJS] and is only done for images that actually need conversion (`background-size` & `mtime`).
+
 [PostCSS]: https://github.com/postcss/postcss
+[PhantomJS]: http://phantomjs.org