chore: update CONTRIBUTING.md and make releaseType node (#240)

Author: bcoe

packages/google-cloud-oslogin/CONTRIBUTING.md

@@ -34,6 +34,7 @@ accept your pull requests.
 1.  Ensure that your code adheres to the existing style in the code to which
     you are contributing.
 1.  Ensure that your code has an appropriate set of tests which all pass.
+1.  Title your pull request following [Conventional Commits](https://www.conventionalcommits.org/) styling.
 1.  Submit a pull request.
 
 ## Running the tests
@@ -46,8 +47,17 @@ accept your pull requests.
 
 1.  Run the tests:
 
+        # Run unit tests.
         npm test
 
+        # Run sample integration tests.
+        gcloud auth application-default login
+        npm run samples-test
+
+        # Run all system tests.
+        gcloud auth application-default login
+        npm run system-test
+
 1.  Lint (and maybe fix) any changes:
 
         npm run fix