Update CONTRIBUTING.md (parse-community#5241)

Author: dplewis

CONTRIBUTING.md

@@ -61,6 +61,7 @@ Once you have babel running in watch mode, you can start making changes to parse
 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)
+* The Postgres adapter has a special debugger that traces all the sql commands. You can enable it with setting the environment variable `PARSE_SERVER_LOG_LEVEL=debug`
 * 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