title: GraphQL: What is rootValue specification that is passed to execute?
body: <p>I use express-graphql which is initialized with <code>rootValue</code> and passes it to GraphQL's <a href="https://github.com/graphql/graphql-js/blob/master/src/execution/execute.js" rel="nofollow noreferrer">execute</a> function. I wonder what is <code>rootValue</code>'s specification. I have seen some simple examples but I'm interested in full specification.</p>