docs: Move SQL schema to README.md, add more help

Move SQL schema guidelines to a dedicated README.md, with all the
niceties of Markdown formatting and the added prominence of GitHub
displaying it whenever navigating to the `src/sql` folder.

Add a brief high-level overview of Quassel's database schema.

Polish up the SQL schema guidelines to follow the current Quassel
workflow.

Clean up remarks in the script files, including pointing to the new
README.md file.