Directives are not on cheat sheet

[joshuadutton]

Directive declaration definition:
graphql/graphql-js#318

To do:

• add directive to Type Definitions
• add a Directive Types section with a declaration and usage examples
• add a directive declaration (one liner) in the What does it look like? example

Maybe we could get rid of the Input with default value and Input with multiple arguments examples (while keeping the Input with multiple arguments and default values) to make space.

You could include GraphQL supported directives, but it may not be worth the space:

• @skip and @include (https://facebook.github.io/graphql/#sec-Type-System.Directives or https://github.com/facebook/graphql/blob/master/spec/Section%203%20--%20Type%20System.md#directives)
• @depricated is supported in the reference implementation and should be added to the spec soon (https://github.com/facebook/graphql/blob/rfc-idl/spec/Section%203%20--%20Type%20System.md#directives)

I would be happy to add this myself, but since this is an image, I'm not sure how I would go about that.