Block access to mentoring if you have no slots remaining

After the before the upgrade to v3, I had 4 exercises waiting for mentor feedback. After the upgrade, I planned on putting them back into the mentor queue, but was hit with
>You have no more mentoring slots available.
You can get more mentoring slots by earning reputation.

but didn't see how much reputation would net me more slots. So I think this prompts the addition of a section for showing what other potential rewards (other than badges) may be out there for those who gather a lot of reputation. I think it's a good motivator for gathering reputation.

At minimum, there should probably be a link explaining how much reputation is needed for more mentoring slots.