Added supported databases to README.md
Added supported databases to README.md

Added links to docs for each backend

Changed the urls from http to https

marked the code block as toml

suggested by @JohnTitor

Co-Authored-By: Yuki Okushi <85857f4af0f1ba28967e979a9055a2f74102998d@gmail.com>

diesel-rs/diesel,diesel-rs/diesel,sgrif/diesel,sgrif/diesel