Document how to make edits

[harej]

The README tells me how to instantiate the API client and how to retrieve basic items of information from them. It does not explain how to make edits.

[derenrich]

this is a good point. for now you can see the unit tests for how to make edits https://github.com/derenrich/wikibase-rest-api-client/blob/master/tests/test_rw_client.py#L105