Improving documentation on JSON serialization

I to a shot at clearing up matters regarding how JSON serialization is supposed to use MTLJSONAdapter instead of going straight to `+[MTLModel modelWithDictionary:error:]`. See #94.

[Click the eggplant for rendered Markdown: :eggplant:.](https://gist.github.com/robb/6321915)

Let me know if this helps and if it needs further changes.

Pinging @jarsen @horseshoe7 for feedback