chore: add introduction to RELEASES.md (#2476)

Author: kim-em

RELEASES.md

@@ -1,4 +1,13 @@
-Unreleased
+# Lean 4 releases
+
+We intend to provide regular "minor version" releases of the Lean language at approximately monthly intervals.
+There is not yet a strong guarantee of backwards compatibility between versions,
+only an expectation that breaking changes will be documented in this file.
+
+This file contains work-in-progress notes for the upcoming release, as well as previous stable releases.
+Please check the [releases](https://github.com/leanprover/lean4/releases) page for the current status of each version.
+
+v4.0.0
 ---------
 
 * [`dsimp` / `simp` / `simp_all` now fail by default if they make no progress](https://github.com/leanprover/lean4/pull/2336).