Add note on depth option (microsoft#17535)

CONTRIBUTING.md

@@ -114,11 +114,12 @@ Once the team have approved an issue/spec, development can proceed. If no develo
 Once you've discussed your proposed feature/fix/etc. with a team member, and you've agreed an approach or a spec has been written and approved, it's time to start development:
 
 1. Fork the repository if you haven't already.
-1. Clone your fork locally.
-1. Create & push a feature branch.
-1. Create a [Draft Pull Request (PR)](https://github.blog/2019-02-14-introducing-draft-pull-requests/).
-1. Work on your changes.
-1. Build and see if it works.
+2. Clone your fork locally.
+> NOTE: Using a shallow clone can significantly speed up the initial clone command.  Pass `--depth=1` to git if you do not need any history in your local repo.
+3. Create & push a feature branch.
+4. Create a [Draft Pull Request (PR)](https://github.blog/2019-02-14-introducing-draft-pull-requests/).
+5. Work on your changes.
+6. Build and see if it works.
 
 ### Testing