Updated documentation (#158)

Author: dstibbe

CONTRIBUTING.md

@@ -80,6 +80,7 @@ See [How to Write a Git Commit Message](https://chris.beams.io/posts/git-commit/
 * Run `./gradlew assemble` to build the project and produce the corresponding artifacts.
 * Run `./gradlew test` to test the module and speed up development.
 * Run `./gradlew build` to build the project, which also runs all the tests.
+* Run `./gradlew allTests` to run all tests.
 
 *note*: when you change the data model, you might need to regenerate the .api files by running `./gradlew apiDump`.