update release notes for EasyBuild v1.16.0

this may need some minor updates before the actual release (e.g. for additional easyconfigs PRs being merged in), but most of the info should be there

best look at the rendered version for reviewing this: http://boegel-eb.readthedocs.org/en/release_notes_EasyBuild-1.16.0/Release_notes.html
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