Document how to test changes in contribution guidelines

[dzirbel]

Since the gradle configuration of this project is non-trivial, I would find it helpful to have a description of how to test changes in the contribution guidelines. I've been running

./gradlew check -x jsIrBrowserTest -x jsLegacyBrowserTest

since my environment doesn't have a headless browser installed to run the JS tests, which works well and includes linting and (I believe) the majority of tests. Perhaps this could be consolidated into its own gradle task, or just added to the guidelines, unless there is a better way.