Created a README.md

The generic http://www.mono-project.com website you put into the GitHub description is not helpful at all. http://www.mono-project.com/Mono.Upnp is empty. Upstream was located somewhere else http://projects.qnetp.net/projects/show/mono-nat but is also dead. There is a modern fork in the makings at https://github.com/lontivero/Open.NAT you may want to direct developers instead.