Update README

We have changed locations since this README was created.  This gives it
a little bit more meat and updates it with the new channel location.
move CONTRIBUTE into README

Both of these files are very short and include information on
contributing.  This puts it all in one spot and once they both have
significant substance we can split them back apart.
Merge pull request #8 from alunduil/update-readme

Update readme
add portage Dockerfile

This Dockerfile should build automatically even on hub.  I'll add this
and setup a cronjob to poke the hub's API daily so we should get daily
portage Docker images.