External contributors often want to know:

 * Why is WebView version a.b.c.d incompatible with Android x.y.z?
 * When will upstream WebView be compatible with Android x.y.z?
 * Which WebView version should I build for Android x.y.z? (this has different answers for development vs. release)

It might be nice to publish info for which milestones we upstreamed each next-layer.

Related (but not a blocker): issue 760187