Add documentation for requesting object byte ranges

This pull request adds examples for using the `Range` HTTP header with objects inside Riak CS. It also suppresses a message citing that this feature will be available in a future release (for all versions greater then `1.3.0`, which is when the feature was introduced). [0]

[0] https://github.com/basho/riak_cs/blob/develop/RELEASE-NOTES.org#additions-3