When this file exists, Debian builds will automatically compare the built
ABI and symbols to the ones defined in the list. If there is a mismatch,
developers need to update the libmariadb3.symbols file, otherwise the build
fails.

This ensures there are no accidental symbol changes. This also helps track
what symbols changed and in what versions.

Also add a README embedded in the sources to facilitate correct use of this
new file.

This change is made for branch 10.2, as it was the first server version to
introduce an embedded libmariadb3 client library, and from this version
the file with the same libmariadb3 specific content will be merged to
10.3, 10.4 and 10.5.

Related: MDEV-21732