Update doc/README

The doc/README was written in markdown, but has not markdown file ending, which
results in Github not recognising it. I changed it and linked the old name to
it, in case some code relies on the old file name.

See Github Issue #33.

Signed-off-by: Erik Bernoth <bernoth@dresearch-fe.de>
Acked-by: Steffen Sledz <sledz@dresearch-fe.de>
Acked-by: Eik Binschek <binschek@dresearch-fe.de>