docs: add GH button in support of Ukraine (#375)

## Summary
Our mission at Meta Open Source is to empower communities through open source, and we believe that it means building a welcoming and safe environment for all. As a part of this work, we are adding this banner in support for Ukraine during this crisis.

README.md

@@ -1,4 +1,4 @@
-# prop-types [![Build Status](https://travis-ci.com/facebook/prop-types.svg?branch=main)](https://travis-ci.org/facebook/prop-types)
+# prop-types [![Support Ukraine](https://img.shields.io/badge/Support-Ukraine-FFD500?style=flat&labelColor=005BBB)](https://opensource.fb.com/support-ukraine) [![Build Status](https://travis-ci.com/facebook/prop-types.svg?branch=main)](https://travis-ci.org/facebook/prop-types)
 
 Runtime type checking for React props and similar objects.