Update CONTRIBUTING.md (#3109)

Note on linting

Author: flovilmart

CONTRIBUTING.md

@@ -9,6 +9,7 @@ We really want Parse to be yours, to see it grow and thrive in the open source c
 * Take testing seriously! Aim to increase the test coverage with every pull request.
 * Run the tests for the file you are working on with `npm test spec/MyFile.spec.js`
 * 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
+* Lint your code by running `npm run lint` to make sure all your code is not gonna be rejected by the CI.
 
 ##### Code of Conduct