The README listed only the SDL2 and the console frontends, and the build
instructions for all of them lived in the README. Now there is a Frontends
section with the eight of them, each linking to a page with how to build it,
how to use it and what it can not do.
The libretro pages take the frontend_ prefix as well. The feature list moves
to doc/features.md, leaving a summary in the README.
The default configuration was wrong in the two places it was written: it
listed a parallel card in slot 1 and a mouse in slot 4, which are empty and a
Mockingboard, and flags -disk35 and -hd that no longer exist. Both lists now
come from -showConfig.
