feat(docs): Update README.md (#1395)
feat(docs): Update README.md (#1395)

Summary:
> Updating Readme.md file. This will make it look a lot cleaner.

### Changes
- [x] Added Flipper logo
- [x] aligning header elements to center

Pull Request resolved: https://github.com/facebook/flipper/pull/1395

Reviewed By: jknoxville

Differential Revision: D22661963

Pulled By: mweststrate

fbshipit-source-id: 01c495da3037a28a9f69907f8f94e42e77143d10

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