documenting the Releases API

- [x] the story about how to create/edit releases 
- [x] how to upload assets
- [x] PR for API refactoring #519 
 - `CreateRelease` -> `Create` in API
 - `EditRelease` -> `Edit` in API
- [x] PR for extension method to transform a `Release` into a `ReleaseUpdate` object #519 
- [x] ensure can you change the release `target_commitish` as part of an update #536