Improving documentation on JSON serialization

I took 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://github.com/github/Mantle/tree/improving-docs#mtljsonserializing)

Let me know if this helps and if it needs further changes.

Pinging @jarsen @horseshoe7 for feedback

Closes #94