Get GitHub to render the markdown in README properly

GitHub requires markdown files to have the proper file extension before rendering them as markdown. Thus, README needs to be renamed to README.md for GitHub to recognize it as markdown. Unfortunately, however, `automake` fails to recognize README files with file extensions as actually being README files, so it would also be necessary to keep a file with the old name around to keep `automake` happy, at least with the `gnu` option.