Update README.md

[honglooker]

This repo is no longer supported, so let's make that crystal clear

[honglooker]

preview:

@haberman lmk if you want any verbiage / md tweaks

[haberman]

I wonder if we should do something more drastic and more immediately obvious:

1. Move all files into an archived/ top-level directory, so the file list is short and the README appears "above the fold."
2. Delete all of the README except for the short remark about the repo being moved.

[honglooker]

Happy to do either -- do you lean either way? For starters, I can nuke the readme except the little blurb.

Does it make sense to archive the repo itself? [1]

[1] https://docs.github.com/en/repositories/archiving-a-github-repository/archiving-repositories

[honglooker]

readme updated @haberman

[honglooker]

abrogated by #1532