Clarify documentation on the required GitHub access token

[datenreisender]

In https://github.com/greenkeeperio/greenkeeper-lockfile/blame/master/README.md#L41 it says:

First create a GitHub access token with push access to your repository and make it available to your CI's environment as GH_TOKEN.

This left me unsure about which scope I should choose on https://github.com/settings/tokens/new to allowgreenkeeper-lockfile to push to my private repo. I assume repo is necessary and sufficient.

If this is correct, it would be helpful, to state this more clearly in the docs.

[jonathanong]

could we just use SSH instead?