doc: Added instructions for how to add an upsteam to forked repo

As a first time git developer, I struggled to understand whether to create a new fork for each pull request or not.

After asking the IRC chat, I have added this to the documentation to further help new developers using git.

Co-Authored-By: Michael <[email protected]>

Author: dannmat

CONTRIBUTING.md

@@ -46,7 +46,7 @@ facilitates social contribution, easy testing and peer review.
 
 To contribute a patch, the workflow is as follows:
 
-  1. Fork repository
+  1. Fork repository ([only the first time](https://help.github.com/en/articles/fork-a-repo)).
   1. Create topic branch
   1. Commit patches