Added warning to set credentials before working with forks

Author: hinyan17

_docs/developer/getting_started/commit_to_PR_from_fork.md

@@ -65,6 +65,13 @@ The instructions below are for command line use of git.
     and review the PR and make code edits and new commits to your
     local branch as needed.
 
+    Note: make sure your git credentials are set before pulling or pushing to a forked 
+    branch. You can do this with the following commands. Use the --global flag if you 
+    want to set this outside of just Submitty.
+    '''
+    git config user.name "Your Name"
+    git config user.email "Your Email"
+    '''
 
 
 5.  In order to push the changes to the contributor's fork (and the PR