title: What are the intended use cases for the resolve function's context and rootValue parameters?
body: <p>The <code>0.5.0</code> release of graphql-js contains a breaking change to the <code>resolve</code> function - it now receives an additional <code>context</code> parameter.</p>
  
  <p>It looks to me like <code>context</code> is intended to carry user session data. What is the use case for the <code>rootValue</code> parameter, now?</p>