Add CircleCI status badge (#4473)

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: #4473

Reviewed By: cbalioglu

Differential Revision: D37052250

Pulled By: dianaml0

fbshipit-source-id: e5e4c38a9108c769953ef2202c7adb8aa335771a

README.md

@@ -7,6 +7,7 @@
   <a href="https://github.com/pytorch/fairseq/releases"><img alt="Latest Release" src="https://img.shields.io/github/release/pytorch/fairseq.svg" /></a>
   <a href="https://github.com/pytorch/fairseq/actions?query=workflow:build"><img alt="Build Status" src="https://github.com/pytorch/fairseq/workflows/build/badge.svg" /></a>
   <a href="https://fairseq.readthedocs.io/en/latest/?badge=latest"><img alt="Documentation Status" src="https://readthedocs.org/projects/fairseq/badge/?version=latest" /></a>
+  <a href="https://app.circleci.com/pipelines/github/facebookresearch/fairseq/"><img alt="CicleCI Status" src="https://circleci.com/gh/facebookresearch/fairseq.svg?style=shield" /></a>
 </p>
 
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