Document periodic maintenance tasks (Stack snapshot update, cache clear, problem-specifications version check)

In #435 we update to lts-7.9.

Is there a rule for how often we want to do this? For example, lts-7.15 is out now.

Or is it just "we update whenever someone feels like it"?

Let's see when a few versions were released since 7.9:

* 2016-11-14
* 2016-11-22
* 2016-11-28
* 2016-12-04
* 2016-12-12
* 2016-12-18
* 2017-01-08

Okay, so that means (normally) weekly. I don't think we particularly want to update every week. So, do we want to adopt some sort of thing like "let's update every month", or do we want it to be looser, like "we'll update if something is broken, otherwise we won't"?