Reader admin docs added

[cjeller1592]

No description provided.

[thebaer]

I think this needs a good bit of reworking. High level to-dos:

• Switch away from the question-answer format in the intro -- instead, maybe directly mention some of the specific ways people use the Reader today
• Add image alt text
• Rework everything to not use so much passive voice. The "Default Visibility" and "Private" sections in particular were really hard for me to read. Instead try active voice, joining sentences together, and generally making things flow more coherently
• Make sure the "Setting Reader as Landing Page" is entirely accurate, especially regarding default pages while logged in

This one wasn't crucial for the 0.12 release, so no rush -- better to get it right. Take your time to understand what info we need to convey for admins, who all uses this feature (and how), and we'll get this into the next iteration.