Rewrite the README so it is closer to the gax-java one
Rewrite the README so it is closer to the gax-java one

This also adds a build status badge, in anticipation of Travis being enabled on the repo very soon :)
I've preserved the details of what's going to be in which package, with the hope that we'll move it
to wider documentation at some point soon.

jskeet/gax-dotnet,googleapis/gax-dotnet,googleapis/gax-dotnet,jskeet/gax-dotnet