docs: add GH button in support of Ukraine (#128) Summary: Our mission at [Meta Open Source](https://opensource.facebook.com/) 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: https://github.com/facebook/FBRetainCycleDetector/pull/128 Reviewed By: sarora5 Differential Revision: D34628567 Pulled By: dmitryvinn-fb fbshipit-source-id: 20ae973e6ee8d5fed935aeb2e0f166cdf7ab1002