docs: Add docs on how to add a test image

Signed-off-by: caleb <etemesicaleb@gmail.com>

docs/AddingATest.md

@@ -0,0 +1,52 @@
+## Adding a new test to the repo
+
+This walkthrough takes one through steps it takes to add a simple test to the library.
+
+### 1. Add image/images to test-images
+
+`test-images` directory is the repo that contains sample images that help test for regressions and correctness.
+
+It contains a set of small/medium-sized images that stress certain parts of the decoder, so in case we have a
+regression we don't want to occur, we add the image to `test-images`.
+
+The structure is `test-images->format->[images]` for ease of use.
+
+- Remember to add the images to VCS
+
+### 2. Add the file details to `zune-tests/tests/[{format}.json]`
+
+E.g there is a `jpeg.json` which contains information about jpeg files.
+
+The format is simple
+
+| Field      | Status    | Explanation                                                                 |
+|------------|-----------|-----------------------------------------------------------------------------|
+| name       | Mandatory | File name with extension                                                    |
+| hash       | Mandatory | xxhash(seed of `0`) result of the decoded result still not in planar format |
+| colorspace | Optional  | Expected colorspace output of the image, only used by `zune-jpeg` currently |
+| comment    | Optional  | A reason why the file exists, useful for debugging purposes                 |                
+
+Here is an example of such a file description
+
+```json
+{
+  "name": "weid_sampling_factors.jpg",
+  "hash": 290923794124024565485763288568005295883,
+  "comment": "Non normal sampling factors, h2v1 for all components",
+  "colorspace": "rgb"
+}
+```
+
+Each entry is part of an array, so it should be inside `[]`
+
+#### Tip: Incase you don't know the hash and you have ensured the output is correct(matches other libraries)
+
+- Run the testing once, once you get the error, copy the hash printed to stdout
+
+### 3. Run decoder
+
+Ensure nothing is failing.
+
+### 4. Profit.
+
+Now we can easily track regressions