restoring meaningful README.md files to the repos

[Dowwie]

If I were new to rust-postgres, I wouldn't know much about it through the front page of github other than it is a workspace of crates. The README.md was the first thing I found when I discovered rust-postgres 2 years ago. The README helped me get started, not the api docs.

[sfackler]

rust-postgres is just a workspace of crates. The details of the individual crates themselves are in their documentation, which is linked in the README. I don't think it's necessary to maintain docs in both places at once.