Add documentation for additional features of app

[misaochan]

There are a few features of the app that don't have much easily-accessed documentation:

• Automatic category suggestions - nearby categories if user submits a geotagged photo, recent categories, and categories based on the title of the submitted photo
• The ability to select 'share' from the external gallery or camera app, which launches our app
• The ability to take a photo from within the app with the camera option in the menu bar

I'm not sure how to add those to the website without causing too much bloat to the current single-page design, though. What do you guys think?