Per wikitech-l discussion: it would be nice if the various DBMSs we support kept...

Per wikitech-l discussion: it would be nice if the various DBMSs we support kept all of their docs in one place. So make a place and move two already existing READMEs there. Add some info to /docs/database.txt about this as well. Postgres, Oracle, Mssql need to add some info kthnx :)