Add documentation for android unit tests (wix#365)

docs/Guide.Contributing.md

@@ -86,6 +86,16 @@ To run the e2e tests, after the application was built.
 npm run e2e
 ```
 
+### Native Tests
+
+We got native tests for our client code, here is how you execute it
+
+#### Android
+
+0. Install Java and Android SDK 25
+1. In `detox/android` run `./gradlew install`
+2. Run `./gradlew test` to run the tests
+
 ### Code Generation
 
 We are using a code generator based on `babel` and `objective-c-parser` to generate a Javascript Interface for `EarlGrey` (the testing library we use on iOS).
@@ -104,4 +114,4 @@ What happens under the hood can be seen in [`generation/`](../generation); it bo
     4. Assemble type checks and return value to complete function
 3. Generate the code for the syntax tree & add helpers
 
-If you would like to extend the code generation, please make sure to read the [`generation/README.md`](../generation#generation)
+If you would like to extend the code generation, please make sure to read the [`generation/README.md`](../generation#generation)