Introducing a CONTRIBUTING guide

There's no `go get` here :wink:

In fact, rather than dictating where people keep their GOPATH here, why don't we replace step 1 with something like: "Configure your $GOPATH and run 'go get' as described in the README", with a link to https://github.com/rackspace/gophercloud/tree/v0.2.0#how-to-install? We can change step 2 to `cd ${GOPATH}/src/github.com/rackspace/gophercloud`.