added instructions for syncing upstream master with forked master via PR

[bdalal]

What does this PR do?

Fixes #8742

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@stas00

[stas00]

@LysandreJik, @sgugger

We are trying to give instructions to avoid:

1. this:
   
   This is just one of the many examples - a snapshot is from the bottom of [s2s test_finetune_trainer] failing multigpu test #8400

   You can browse recent PRs for many more of these.

   These are not "legit" references - but automatic replays of PRs.

2. unnecessary notifications for the developers mentioned in PR commit messages when these are replied in user forks.

CONTRIBUTING.md is not the most intuitive place for this, but at the moment there is no other place I could think of. At the very least if a new fork user starts doing this, we can refer them to this section.

Of course, the perfect solution would be for github to give repo admins an option to not allow ping-backs from repo forks. But I don't think it's available now.

[sgugger]

LGTM, thanks for adding this!

[LysandreJik]

LGTM