README for using xFormers in FairSeq (#4402)
README for using xFormers in FairSeq (#4402)

Summary:
# Before submitting

- [ ] Was this discussed/approved via a Github issue? (no need for typos, doc improvements)
- [ ] Did you read the [contributor guideline](https://github.com/pytorch/fairseq/blob/main/CONTRIBUTING.md)?
- [ ] Did you make sure to update the docs?
- [ ] Did you write any new necessary tests?

## What does this PR do?
Fixes # (issue).

## PR review
Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

## Did you have fun?
Make sure you had fun coding �

Pull Request resolved: https://github.com/pytorch/fairseq/pull/4402

Reviewed By: xwhan

Differential Revision: D36208103

Pulled By: dianaml0

fbshipit-source-id: 1600356d20dc32340935c0c88c1f700a1cdefa14

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq