readme: add "Status" section and stop saying it's not ready for use

I have been using the tool myself for 7-8 months now, and the UX has
improved substantially, so I think it's ready for use.

README.md

@@ -4,9 +4,9 @@
 ## Disclaimer
 
 This is not a Google product. It is an experimental version-control system
-(VCS). It is not ready for use. It was written by me, Martin von Zweigbergk
-(martinvonz@google.com). It is my personal hobby project. It does not indicate
-any commitment or direction from Google.
+(VCS). It was written by me, Martin von Zweigbergk (martinvonz@google.com). It
+is my personal hobby project. It does not indicate any commitment or direction
+from Google.
 
 
 ## Introduction
@@ -68,6 +68,16 @@ Features:
    Jujutsu keep commits rebased even if there are conflicts.
 
 
+## Status ##
+
+The tool is quite feature-complete. I have almost exclusively used `jj` to
+develop the project itself since early January 2021. However, there *will* be
+changes to workflows and backward-incompatible changes to the on-disk formats
+(I'll try to provide upgrade commands if requested). It's also likely that
+workflows and setups different from what I personally use are not well
+supported. 
+
+
 ## Tutorial
 
 This text assumes that the reader is familiar with Git.