How do I add a description to a field in “GraphQL schema language”

If you’re using GraphQL.js version 0.7.0 or above, you can simply add a comment directly before the field, type, or argument you want to describe. For example:

# A type that describes the user
type User {
     # The user's username, should be typed in the login field.
     username: String!
     # The user's password.
     password: String!
}

Below version 0.7.0 it is not possible to add descriptions inside the schema language.

UPDATE: since version v0.12.3 you should use string literals

"""
A type that describes the user. Its description might not 
fit within the bounds of 80 width and so you want MULTILINE
"""
type User {
     "The user's username, should be typed in the login field."
     username: String!
     "The user's password."
     password: String!

}