Updated README

Updating the README with new information and code samples. Also added contributing and help sections. We will need to update some links if and when we update the main homepage - but this can be done in another PR.

You can see a preview of rendered markdown [here](https://github.com/jamiehannaford/gophercloud/tree/new-readme#gophercloud-the-openstack-sdk-for-go)  :smile: