docs: Improves explanations on Postgres testing (#3873)

* Update CONTRIBUTING.md

* Update CONTRIBUTING.md

Author: flovilmart

CONTRIBUTING.md

@@ -10,7 +10,19 @@ We really want Parse to be yours, to see it grow and thrive in the open source c
 * 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.
+* Never publish the lib folder.
+
+##### Run your tests against Postgres (optional)
+
+If your pull request introduces a change that may affect the storage or retrieval of objects, you may want to make sure it plays nice with Postgres.
+
+* Run the tests against the postgres database with `PARSE_SERVER_TEST_DB=postgres npm test`. You'll need to have postgres running on your machine and setup [appropriately](https://github.com/parse-community/parse-server/blob/master/.travis.yml#L37)
+* If your feature is intended to only work with MongoDB, you should disable PostgreSQL-specific tests with:
+   
+  - `describe_only_db('mongo')` // will create a `describe` that runs only on mongoDB
+  - `it_only_db('mongo')` // will make a test that only runs on mongo
+  - `it_exclude_dbs(['postgres'])` // will make a test that runs against all DB's but postgres
 
 ##### Code of Conduct
 
-This project adheres to the [Open Code of Conduct](http://todogroup.org/opencodeofconduct/#Parse Server/[email protected]). By participating, you are expected to honor this code.
+This project adheres to the [Contributor Covenant Code of Conduct](https://github.com/parse-community/parse-server/blob/master/CODE_OF_CONDUCT.md). By participating, you are expected to honor this code.