Include NEWS in documentation again

This was implemented in 350ecfec and reverted in 700bb843
after it broke “make distcheck”. With other changes in this
patch series this will work now.

Contributing to the original problem was that the news.rst file
was not distributed. When we distribute the build documentation,
the source must also be included (see Automake manual).

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>