* README -> README.md + OVERVIEW.md
* meson.build: Install README.md and OVERVIEW.md
* Makefile.am: Distribute and install README.md and OVERVIEW.md
* configure.ac: Set `foreign` option to automake.
Without the `foreign` option, old versions of automake require
a README file, which no longer exists. New versions (like 1.16.5)
without the `foreign` option require README or README.md.