Add initial timeline proposal

Signed-off-by: Geoffrey Biggs <gbiggs@killbots.net>

source/Releases/Release-Iron-Irwini.rst

@@ -56,4 +56,31 @@ To come.
 Release Timeline
 ----------------
 
-To come.
+    Mon. April 10, 2023 - Alpha + RMW freeze
+        Preliminary testing and stabilization of ROS Base [1]_ packages, and API and feature freeze for RMW provider packages.
+
+    Mon. April 17, 2023 - Freeze
+        API and feature freeze for ROS Base [1]_ packages in Rolling Ridley.
+        Only bug fix releases should be made after this point.
+        New packages can be released independently.
+
+    Mon. April 24, 2023 - Branch
+        Branch from Rolling Ridley.
+        ``rosdistro`` is reopened for Rolling PRs for ROS Base [1]_ packages.
+        Iron development shifts from ``ros-rolling-*`` packages to ``ros-iron-*`` packages.
+
+    Mon. May 1, 2023 - Beta
+        Updated releases of ROS Desktop [2]_ packages available.
+        Call for general testing.
+
+    Mon. May 15, 2023 - Release Candidate
+        Release Candidate packages are built.
+        Updated releases of ROS Desktop [2]_ packages available.
+
+    Thu. May 18, 2023 - Distro Freeze
+        Freeze rosdistro.
+        No PRs for Iron on the ``rosdistro`` repo will be merged (reopens after the release announcement).
+
+    Tue. May 23, 2023 - General Availability
+        Release announcement.
+        ``rosdistro`` is reopened for Iron PRs.