Better document how to implement custom FieldValidators

[smyrman]

As illustrated by #125, the documentation on creating custom schema validators could probably be improved somewhat in the main README.

Should cover:

• What a Validator could get of input (rest/graphQL values or db values), and what it should return (db value)
• What a Serializer, if implemented may get as input (db values), and what it should return (easily JSON serializable types).
• Move the section related to support JSONSchema through custom validayors.