README: fill out some useful quickstart information

The README file is usually the first thing consulted when a user
or developer obtains a copy of the QEMU source. The current QEMU
README is lacking immediately useful information and so not very
friendly for first time encounters. It either redirects users to
qemu-doc.html (which does not exist until they've actually
compiled QEMU), or the website (which assumes the user has
convenient internet access at time of reading).

This fills out the README file as simple quick-start guide on
the topics of building source, submitting patches, licensing
and how to contact the QEMU community. It does not intend to be
comprehensive, instead referring people to an appropriate web
page to obtain more detailed information. The intent is to give
users quick guidance to get them going in the right direction.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <1444671679-17674-1-git-send-email-berrange@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>