docs: Adding local testing steps to contributing guide (googleapis#1743)

* chore: Adding local testing steps to contributing guide

* Update CONTRIBUTING.md

Co-authored-by: Benjamin E. Coe <[email protected]>

Author: ddixit14

CONTRIBUTING.md

@@ -55,4 +55,25 @@ accept your pull requests.
 
         npm run fix
 
-[node]: https://nodejs.org/en/
+## Testing a new feature using CLI
+
+1. After you've written some new code, in order to test it out, you can use the [CLI][CLI].
+
+   The below command should be run from the root of the source code:
+
+   ```
+   node build/src/bin/release-please.js release-pr \
+   --token=$GITHUB_TOKEN \
+   --repo-url=<owner>/<repo> [extra options]
+   ```
+   
+   It is equivalent to running the CLI command:
+
+   ```
+   release-please release-pr \
+   --token=$GITHUB_TOKEN \
+   --repo-url=<owner>/<repo> [extra options]
+   ```
+
+[node]: https://nodejs.org/en/
+[CLI]: https://github.com/googleapis/release-please/blob/main/docs/cli.md/