More high-level motivation in README.md

[21echoes]

something like flask's, e.g.:

• What is Cartographer?
  Cartographer Python library that uses the JSON API specification to build an API, especially with Flask. Cartographer makes it easier for developers to quickly build an API utilizing JSON API. Using JSON API enables developers to build clients that can take advantage of this standardization, by using existing client libraries that leverage caching, etc.
• Is it ready?
  etc.

[skarger]

I have a tangentially-related question which might be helpful as part of the high-level motivation summary:

How does cartographer compare to https://marshmallow.readthedocs.io/en/latest/?
What was the motivation to build cartographer rather than using marshmallow?

In one of my projects I've been facing the question of whether to use marshmallow or something more customized to json:api.