added offsetToCursor to documentation

Author: ryanblakeley

README.md

@@ -61,6 +61,8 @@ it takes a promise that resolves to an array, and returns a promise that
 resolves to the expected shape by `connectionType`.
  - `cursorForObjectInConnection` is a helper method that takes an array and a
 member object, and returns a cursor for use in the mutation payload.
+- `offsetToCursor` takes the index of a member object in an array and
+returns a cursor for use in the mutation payload.
 
 An example usage of these methods from the [test schema](src/__tests__/starWarsSchema.js):