Improve README + Docs

[Cito]

• The README should have some explanation what this package does - "GraphQL Server core package" is not really very informative.

• It should also list which classes/functions are part of the API (HttpQueryError, default_format_error, encode_execution_results, json_encode, load_json_body, run_http_query)

• The functions that are part of the official API should all have docstrings.

• The list of packages that use graphql-server-docs needs to be updated, too

  • graphene-django does not use graphql-server-core at all, contrary to what the README says
  • in addition to flask-graphql, sanic-graphql and webob-graphql, the following packages use it:
  • aiohttp-graphql
  • graphene-sqlalachemy (in flask example)
  • kennethreitz/responder (in graphql extension)
  • moritzmhmk/wsgi-graphql

[Cito]

This is now all covered in PR #21.