add Snyk announcement

Author: espy

README.md

@@ -1,3 +1,7 @@
+***
+☝️ **Important announcement:** Greenkeeper will be saying goodbye 👋 and passing the torch to Snyk on June 3rd, 2020! [Find out how to migrate to Snyk and more at greenkeeper.io](https://greenkeeper.io)
+***
+
 # Badges for Greenkeeper
 
 [![Greenkeeper badge](https://badges.greenkeeper.io/greenkeeperio/badges.svg)](https://greenkeeper.io/)