add history notes to README.md

Hi @adam-mercer , it is awesome to see the whole history back in place. Thanks so much!

Here's a suggested README update to clarify the status of old commit hashes and references to Redmine issues. Of course, feel free to adapt or reword as you see fit.

Also sorry for this being a branch in lscsoft/lalsuite itself, which I guess is against our policy? I had expected when editing the README through the web interface I'd get an option to fork, too; but either I must have clicked blindly past it or, given my master access, it short-circuits that route. I should probably just avoid going that "easy" way in the future.