[Doc] Update CONTRIBUTING.md by adding command for running tests to P…

…R directions (opensearch-project#335)

* Add command for running tests to PR directions

Signed-off-by: Nicholas Ung <nicholasung22@gmail.com>

* Add more details about -u tag

Signed-off-by: Nicholas Ung <nicholasung22@gmail.com>

* Add reason for snapshot updates

Signed-off-by: Nicholas Ung <nicholasung22@gmail.com>

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Signed-off-by: Nicholas Ung <nicholasung22@gmail.com>

CONTRIBUTING.md

@@ -31,7 +31,7 @@ To send us a pull request, please:
 
 1. Fork the repository.
 2. Modify the source; please focus on the specific change you are contributing. If you also reformat all the code, it will be hard for us to focus on your change.
-3. Ensure local tests pass.
+3. Ensure local tests pass by running `yarn test -u`. Snapshot files must always represent the current state of the code they are covering for proper testing to occur. The `-u` flag is an alias for the `--updateSnapshot` option, which automatically updates any snapshots that have been affected by your changes.
 4. Commit to your fork using clear commit messages.
 5. Send us a pull request, answering any default questions in the pull request interface.
 6. Pay attention to any automated CI failures reported in the pull request, and stay involved in the conversation.