Document that SSH executable is required for SSH repo operations

[stephen-dexda]

This isn't mentioned in https://github.com/gitpython-developers/GitPython/blob/master/README.md#requirements or https://gitpython.readthedocs.io/en/stable/intro.html#requirements. It's sort of implied by the fact that GitPython is relying on git which in turn relies on ssh, but would be useful to make explicit.

[Byron]

Thanks for pointing it out! We would be grateful for a PR that adds the parts that you would have wanted to see.