meta: add note about email sync to CONTRIBUTING.md

PR-URL: #16340
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>

Refs: #16339 (comment)

CONTRIBUTING.md

@@ -242,6 +242,10 @@ who you are:
 $ git config --global user.name "J. Random User"
 $ git config --global user.email "j.random.user@example.com"
 ```
+Please make sure this local email is also added to your
+[GitHub email list](https://github.com/settings/emails) so that your commits
+will be properly associated with your account and you will be promoted
+to Contributor once your first commit is landed.
 
 #### Step 1: Fork