All the info to return codes from the API functions are available in the `README.md` file as suggested.

The API documentation of merge requests contains info to status codes for all functions.
Required arguments are now checked in the merge requests API functions and a `400 Bad Request` error is
returned if they are not given.

The issues documentation is updated with infos to status codes and the deprecated `DELETE` function and
how to close an issue. A few more tests added to check status codes of API functions.