Added resolver-returned value to the error object, plus further docum…

…entation on expected error fields (graphql#590)

README.md

@@ -118,6 +118,12 @@ The `graphqlHTTP` function accepts the following options:
  errors produced by fulfilling a GraphQL operation. If no function is
  provided, GraphQL's default spec-compliant [`formatError`][] function will be used.
 
+ Although the fields on the error provided to this function may vary (based on the error type), they will typically include:
+ * **message:** the error message
+ * **locations:** the location(s) in the Javascript code where the error ocurred
+ * **path:** the path within the returned results where the error occurred (eg. in a schema violation error, the path to the part of the results that violated the schema)
+ * **result:** the value which was returned by the resolver (if any)
+
 - **`customParseFn`**: An optional function which will be used to create a document
  instead of the default `parse` from `graphql-js`.