MANIFEST.in: include README.mkd and asciidoc files

When `python setup.py sdist` is run to generate a source tarball for
`rpmbuild`, it currently doesn't include some key files which causes
`rpmbuild` to fail. So add these to `MANIFEST.in`.
Merge branch 'add-asciidoc-to-sdist' of https://github.com/grosskur/tito into grosskur-add-asciidoc-to-sdist
Add more missing documentation to MANIFEST.in.