Add course author notes for 7/16 release

@mhoeber @lamagnifica Please review. Draft at http://draft-release-notes-7-16.readthedocs.org/en/latest/07-16-2014.html.

I didn't include the following; let me know if I should:

- https://github.com/edx/edx-platform/pull/4095 (creates an Open edX footer)

- https://github.com/edx/edx-platform/pull/4172 (e-commerce enhancements - I don't think these are externally visible yet)

- https://github.com/edx/edx-platform/pull/4353 and https://github.com/edx/edx-platform/pull/4368 (these two are specific to annotations, which I think only Harvard uses and we don't want to publicize)
OK, changing to "In open response assessments, instructors can now create assignments that allow students to upload an image file as well as write a text response."