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

@ndonald2 I'd add that here, if there are only two (`backgroundManagedObject` and `temporaryContext1`)