docs: add GH button in support of Ukraine

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.

Pull Request resolved: facebookresearch#4010

Reviewed By: wat3rBro

Differential Revision: D34588217

Pulled By: dmitryvinn-fb

fbshipit-source-id: 093a3ac336dbab3bd3b5b75cc724e0c274911438

README.md

@@ -1,5 +1,9 @@
 <img src=".github/Detectron2-Logo-Horz.svg" width="300" >
 
+<a href="https://opensource.facebook.com/support-ukraine">
+  <img src="https://img.shields.io/badge/Support-Ukraine-FFD500?style=flat&labelColor=005BBB" alt="Support Ukraine - Help Provide Humanitarian Aid to Ukraine." />
+</a>
+
 Detectron2 is Facebook AI Research's next generation library
 that provides state-of-the-art detection and segmentation algorithms.
 It is the successor of