[Documentation:Developer] set git credentials working with forks (#655)

**Summary:**
Added a small warning in the "Commit to PR from fork" page that tells
developers to ensure that their git credentials are set before pulling
and pushing from forks. If credentials are not set, then some commands
will fail. This change would help newer developers who might not be
proficient with git.

**Testing**
- Verified that changes appeared on the page
- Format looks good

Author: hinyan17

_docs/developer/getting_started/commit_to_PR_from_fork.md

@@ -40,7 +40,15 @@ The instructions below are for command line use of git.
     ```
 
     Additionally, find the name of the fork on the fork's page on Github.  
-    The `fork_name` may simply be `Submitty`, or the author may have chosen to another name.
+    The `fork_name` may simply be `Submitty`, or the author may have chosen another name.
+
+    Note: make sure your git credentials are set before pulling or pushing to a forked 
+    repo. 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"
+    ```
     
 
 3.  Next, make a local branch on your computer in your working repository