Restructure README.md

These changes make the README more consistent with Compat's README.
Fix Package-Requires header

Thanks to Stefan Monnier for noticing.
Have vc-backup-diff compare adjacent versions by default
Fix thread-last indentation

This was changed with 28.1, so fixing this here will avoid unnecessary
whitespace changes in the future.
Add "Package-Requires" header
Change maintainer address to public inbox
Bump version to 1.1.0
Mention file size in vc-log
Add FIXME comment to vc-backup--get-real
Add Maintainer tag to package header
Mention that vc-backup is autoloaded
Print entire file name in version log
Rename vc-backup--get-read to vc-backup--get-real
Add URL header to repository
Untabify vc-backup.el
Just use a fixnum in vc-backup-diff.
Rename local variables in vc-backup-print-log
Print the user who owns the file