[zulip] Add a note to the README about these Zulip versions.

README.md

@@ -1,3 +1,19 @@
+# React Native · Zulip version
+
+This is a version of React Native maintained by
+[the Zulip project](https://github.com/zulip/zulip),
+for use in the
+[Zulip mobile app](https://github.com/zulip/zulip-mobile).
+
+For more information about this repo,
+[see our documentation](https://github.com/zulip/zulip-mobile/blob/master/docs/architecture/our-rn.md).
+
+---
+
+### Upstream README follows.
+
+---
+
 <h1 align="center">
   <a href="https://reactnative.dev/">
     React Native