added instructions for syncing upstream master with forked master via PR
Author: bdalal
Pull request: yes

# What does this PR do?

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Fixes #8742 

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
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      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


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Comments:
@LysandreJik, @sgugger 

We are trying to give instructions to avoid:

1. this: 
![snapshot_3](https://user-images.githubusercontent.com/10676103/100027351-77edf480-2da1-11eb-8d0a-4590569042c0.png)
This is just one of the many examples - a snapshot is from the bottom of https://github.com/huggingface/transformers/pull/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.