Clarify "push access" in "create a GitHub access token"

[guyellis]

In the readme you have:

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

Following that link and clicking Generate new token I'm presented with:

To me it's not obvious what I should be selecting to provide "push access" to my GitHub repo.

[janl]

we’ll improve this, for the moment: everything under repo

[betaorbust]

If I'm only using greenkeeper for public repos, can I just give access to public_repo?

[AurelioDeRosa]

Happy to submit a PR if this can help in moving things forward.