Document ci-statuses require setting push-remote

[danielcompton]

Add documentation to explain that you need to set the push-remote for Magit to pick up the status checks for a branch.

Taken from https://github.com/vermiculus/magithub/issues/334#issuecomment-394329415.

[vermiculus]

Good addition, thanks. I would probably word it as:

Note that the branch must have a push-remote set in order to find the correct status to use.

[danielcompton]

Fixed.

[vermiculus]

Thanks!