Support String literal descriptions

[landau]

Good day. Thanks for your work on this project. It's incredibly useful.

As per https://github.com/graphql/graphql-js/blob/master/src/utilities/extendSchema.js#L47, descriptions defined within an IDL (SDL) are defined as String Literals. I'm currently seeing the following error.

 ✗ Syntax Error GraphQL (5:1) Unexpected String "My description"

4:
5: "My description"
   ^
6: type QueryRoot {

Thanks for your time.