GitHub releases: describe reproducibility. Markdown.

GitHub released their Releases feature, but our release tarballs are not
simply tar'ing up whatever's in the top-level: we run `make tarball` to
generate metadata used for constructing the real binary and a man-page,
plus a ChangeLog.  This depends upon Git metadata.  So describe how to
reproduce our real tarballs, to avoid user confusion.

Converted README to README.md to get links, etc.  It was very close
already.