Add instructions for syncing forked masters to avoid references
# 🚀 Feature request

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## Motivation

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Ticket created based on discussion https://github.com/lucidworks/transformers/commit/46b17d206529206848116fd6219643446bac938c#commitcomment-44356945
The problem is when someone on a forked repository decides to sync up their masters with upstream (HF master) using a branch and a PR, all the PR and issue references on the upstream will make their way into the forked PR's commit history, if that user creates a merge commit. 
Since GitHub autolinks issues and PRs on public forks, this will end up pinging the devs responsible for the referenced PRs creating unnecessary noise. The solution is to use a squashed merge.

One way to educate users with forked repos about this potential issue is to add instructions on how to do so to the `CONTRIBUTING.md` file.

## Your contribution

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A PR with instructions to avoid this situation will be up shortly.

cc @stas00 