Remove HTML comments from release notes

... and mention two recently published libraries for 2.11

Removing comment in 2014-04-08-release-notes-2.11.0-RC4.md was the
minimal change needed to fix the layout bug on the homepage.

I have updated our generator to strip out internal comments
before shipping the markdown your way:

  https://github.com/scala/make-release-notes/pull/86