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)
Add that a new chapter on participating in discussions has been added. 
>> In addition, the `Discussions`_ chapter has been updated to reflect recent enhancements to the user interface.