README draft

@mgorbach 

Draft of the README.

**NOTE: This is a pull request to merge into #8. If that PR is merged first, this will need to be reopened into `development`**.

#### Related Issues
Potentially closes #2.

#### TODO
- Add link to CocoaDocs page when podspec is submitted

Yep, no problem. There is also `backgroundManagedObjectContext` - should I add that too? I was planning on keeping those things only in the CocoaDocs, as they are less common use cases, but maybe it's better to have it here too.