add documentation about the maintenance job(s)

[tilsammans]

See #861 for the actual change.

[coveralls]

Coverage remained the same at 99.204% when pulling 149e64c on scrollytelling:doc-prune_auto_snapshots_jobs into 40cda2b on codevise:master.

[aviav]

I see the point of adding documentation for the task. I'd rather add it to the Troubleshooting section [1] of the Wiki, because needing the task is contingent on both a) an active user base and b) storage space running out, and this advice shoots trouble (see b)).

README.md up until now seems to me like a place for less situational info. People who manage to grow an active userbase for Pageflow will at some point notice the Troubleshooting section. I'd be surprised if this happens only after they hit a storage limit for their database.

In terms of content, I approve this. If you move it to the Wiki, maybe the reference to Whenever can be left out to leave this deployment choice completely open. This would resemble our practice of not mentioning specific optional deployment choices in the documentation to decrease the future amount of stale docs. Good job in any case!

[1] https://github.com/codevise/pageflow/wiki/Troubleshooting

[tilsammans]

Totally fine with the Wiki; not on board with the Troubleshooting section. To me that reads more like, "I have this error message now what do I do?". So I added a new page: https://github.com/codevise/pageflow/wiki/Maintenance

[aviav]

Even better!