Update documentation for crosapi README.

Change-Id: Iedb3ca5f542c87634870c6c0d22c6604b4fdd734
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2316787Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Commit-Queue: Erik Chen <erikchen@chromium.org>
Auto-Submit: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791309}

chromeos/crosapi/README.md

@@ -3,3 +3,10 @@ communication protocol between lacros (web-browser on ChromeOS) and ash
 (user-space system executable on ChromeOS) for all new IPCs. Some existing IPCs
 might use Wayland or D-Bus to avoid unnecessary rewrites.
 
+The ChromeOS API is eventually going to be stabilized and will need to tolerate
+several milestones worth of version skew between lacros and ash. In the long
+term, these interfaces will potentially need to support years of version skew.
+This means that any mojom files and their transitive dependencies must be
+relatively stable and backwards compatible. By default, mojom files in this
+directory should not include mojoms from any other directories unless they are
+marked \[Stable\].