Update README.md (#690)
Update README.md (#690)

Summary:
Added Apache 2.0 Licence badge to the Readme File

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The motivation was to add the Apache 2.0 Licence badge to the readme file to make it look good and so that users can see it at first instead of scrolling to the bottom

## Changelog

Added Licence Badge to Readme File

Pull Request resolved: https://github.com/facebook/litho/pull/690

Test Plan:
Adds  this badge [![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://github.com/facebook/litho/blob/master/LICENSE)

![image](https://user-images.githubusercontent.com/23498248/90316065-94c29280-df3d-11ea-8235-5c718231e570.png)

Reviewed By: colriot

Differential Revision: D23648341

Pulled By: andrewpmsmith

fbshipit-source-id: d34321ef862d8d31d18690571c5dc54636ea9aaf

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