Add instructions for version and group properties

You might want to lay it out differently but I think users would find it helpful to know how the group and version are set.

In addition some instructions about signing properties would be useful. I found Gabriele Mariotti's blog post helpful http://gmariotti.blogspot.com/2013/09/publish-aar-file-to-maven-central-with.html but some pointers in your README could save some time. Thoughts? I can send another PR for this.