Add support for adding description to schema

The GraphQL spec was amended to allow descriptions for the schema itself, see https://github.com/graphql/graphql-spec/pull/466. I am happy to accept a PR that adds this feature.  Since we mirror `graphql-js` when possible, the easiest route to implementing this is to follow https://github.com/graphql/graphql-js/pull/2419