doc(readme): deprecation warning on `rolling` branch (#3489)

docs(readme): deprecate rolling branch

README.md

@@ -1,3 +1,9 @@
+# :warning: This branch is no longer maintained :warning:
+
+This `rolling` branch has been deprecated.
+Rolling/nightly releases are now based on the `master` branch.
+All new work should forked from the latest changes on `master`, and PRs should be targeted at `master`.
+
 ![lunarvim_logo_dark](https://user-images.githubusercontent.com/59826753/159940098-54284f26-f1da-4481-8b03-1deb34c57533.png)
 
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