User stories for git-pr

@charlierudolph @allewun adding user stories that describe end-user value
There is no way to make it work, other than moving your repository to Github or Bitbucket. 
We don't tell the user to do that and I don't think we should. 
We are just providing an error message letting them know.
We could add a message like "If you would like another hosting service supported, please open an issue at..."

```
As a developer trying to create a pull request for a repository on an unknown hosting service
I should get an error that my hosting service is not supported
So I know why the command does not work
``` 

Change unknown to unsupported on the first line. Then two updated versions of the next lines.