Introducing a CONTRIBUTING guide

I feel like these more mechanical instructions belong at the very top. I'd also like to be more explicit about the git setup, which is slightly different for Go projects than for standard GitHub fare:

 1. Use `go get` to clone the upstream repository;
 2. cd into the repository under your `$GOPATH`;
 3. shuffle around git remotes. `git remote rename origin upstream`, `git remote add origin git@github.com/me/gophercloud`;
 4. `git checkout v0.2.x` to switch to the current development branch;
 5. `git checkout -b my-sweet-feature` to create a new branch for your development work.

You alluded to this with "no fundamental differences" bit, but there *is* extra setup that you need to do to use a Go project off of a fork, and I see it as a point of confusion that could use extra reinforcement.