initial draft

@mhoeber , this is an early take as I am waiting on replies from a couple of people, but I'd appreciate your sense of whether I've missed anything that's on the release page: https://edx-wiki.atlassian.net/wiki/display/ENG/Release+Page%3A+2014-06-24
Since these notes face course authors, could this be rephrased to something like "XBlocks that have been accepted into the edX platform can now be added to courses as...." Specifically, I worry that people who are interested in writing/contributing XBlocks may not understand all the steps they need to take before seeing something on edx.org. https://github.com/edx/edx-platform/blob/master/CONTRIBUTING.rst