Hooks: add a little bit of user documentation

I wasn't sure how the system worked, so I poked around a bit and came up with
a description, with a little help from Neil.
Add hooks to modify Twitter Card attributes

We'd like to customize the Twitter Cards for certain subreddits - use an app
card for /r/IAmA, maybe an embed card for picture/video-heavy subreddits.  This
allows us to write hooks that can do that.