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

The notes API documentation updated with return codes. API now returns `400 Bad Request` if
required attributes are not present. Return codes are documented now, also tested in added tests.
The documentation now reflects the current state of the API.

A few fixes in the notes URI pattern (`notes` instead of `:notes`), also updated
the information to status codes. If `body` attribute is missing from a POST request
a status code 400 (Bad request) is returned. This reflects the code changes from
the previous commit.