Restore ECOSYSTEM.md as Foundation redirect (#58048)

Summary:
Pull Request resolved: #58048

Restore `ECOSYSTEM.md` so existing GitHub links continue to resolve. Explain that React Native is now part of the React Foundation and direct readers to the Foundation website for current ecosystem information.

Changelog: [Internal]

___

Reviewed By: vzaidman

Differential Revision: D116920238

fbshipit-source-id: 5eb49de204fe32fb2eafc22e441d9de846858a1a

Author: cortinico

ECOSYSTEM.md

@@ -0,0 +1,5 @@
+# The React Native Ecosystem
+
+The React Native project is now part of the [React Foundation](https://react.foundation/).
+
+The ecosystem information previously maintained in this document has been superseded by the [React Foundation website](https://react.foundation/). Visit the website for current information about the React Native ecosystem.