Document setting up scheduler in heroku docs

[astrofrog]

Basically one needs to add the Heroku Scheduler add-on (which is free) and then set up jobs such as:

check-stale-issues --repository astropy/astropy --warn-seconds 13392000 --close-seconds 16070400

and

check-stale-pull-requests --repository astropy/astropy --warn-seconds 13392000 --close-seconds 2678400

I think running them daily makes sense, and they should be set for different times.

This should go in doc/heroku.rst