doc: use README as doxygen main page overview

doc: use README as doxygen main page overview

The doxygen page looked a bit dead, the README fills it nicely, and is
already written in the markdown format which doxygen uses (I think?).

Unfortunately the USE_MDFILE_AS_MAINPAGE doxygen config doesn't seem to
do anything.. So we just add a {#mainpage} tag at the top of the README
which isn't so bad. BUT we still need some config option (the
no_extension=md part) so that doxygen will accept README instead of
README.md or somesuch. And that requires an even newer release, 1.8.3.1,
released 2013-01. But if an older version is used, it doesn't spew out
warnings but just skips the README, which is fine.

Signed-off-by: Ran Benita <ran234@gmail.com>