Document supported GraphQL elements

[raminqaf]

We should consider adding a section in our documentation where we address the supported GraphQL elements. For example, we are not supporting GraphQL Schemas with Union or Interface types.

[DawidNiezgodka]

Maybe something along these lines:

Currently, Quick supports the following GraphQL types:

1. The mandatory Query type and the optional Mutation type,
2. Basic Types with field declarations as well as type modifiers, for example: Type Character {name:String!, appearsIn:[Episode!]},
3. All scalar types: Int, String, Float, Boolean, ID together with field's arguments, for example: total(moreThan: Int = 50): Float,
4. Enumerations,
5. Inputs.

Thus, the following types are not supported:

1. Interfaces,
2. Unions,
3. Custom scalars, for example: scalar MyCustomScalar.