Rename Changelog to tag it as markdown, delete duplicate readme file.
At present the Changelog viewed on https://gitlab.com/libxc/libxc/blob/master/ChangeLog doesn't previes as markdown; this is fixed by adding the .md suffix.
There's both README and README.md, the latter appears to be more up-to-date. Duplication is bad, so let's just remove the ASCII file.
Edited by Susi Lehtola