Update CONTRIBUTING.md

Author: drew-gross

CONTRIBUTING.md

@@ -6,8 +6,9 @@ We really want Parse to be yours, to see it grow and thrive in the open source c
 
 ##### Please Do's
 
-* Please write tests to cover new methods.
-* Please run the tests and make sure you didn't break anything.
+* Take testing seriously! Aim to increase the test coverage with every pull request.
+* Run the tests for the file you are working on with `TESTING=1 ./node_modules/jasmine/bin/jasmine.js --filter="MyFile"` (from repo root)
+* Run the tests for the whole project and look at the coverage report to make sure your tests are exhaustive by running `npm test` and looking at (project-root)/lcov-report/parse-server/FileUnderTest.js.html
 
 ##### Code of Conduct