Modify README.md to explain code move to concurrent_ruby gem.

[keithrbennett]

I knew of the thread_safe gem several years ago. Someone on StackOverflow asked what to use for a threadsafe hash. I googled thread_safe to check to see if it was still available and found this repo, and recommended its use based on what I found. There was no indication that I could find to tell me that it was inactive and that I should use concurrent_ruby instead. This README change makes it obvious to anyone viewing it that they should use the concurrent_ruby gem instead of this code so that others won't make the same mistake.

[jdantonio]

Thank you for doing this. I have been meaning to deprecate this gem the way we deprecated atomic but I wanted the merge to prove stable in Rails, first. You are correct that some notification of the merge is necessary.