Merge #17157: doc: Added instructions for how to add an upsteam to forked repo

f09ba06 doc: Added instructions for how to add an upsteam to forked repo (dannmat)

Pull request description:

  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.

ACKs for top commit:
  fanquake:
    ACK f09ba06 - For such a simple change, I think we've bike-shed this enough already. The `bitcoin/bitcoin` repo isn't really where anyone should be learning how to use `git` etc, but I think linking out here is ok.

Tree-SHA512: e0e9d655d0725e0128673afedb81dc5ba9387968fcbb681de7e50155a2cfa1a7f39fad040b596f4de9ad6727a1a8a90fd3d36eaa5242bc12186c3b82abd23fb2

Author: fanquake

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