Documentation updates

* doc/source/index.rst: pbr requires git tags to be signed for using it
  to generate version information. Added a note mentioning this
* doc/source/packagers.rst: documented SKIP_GENERATE_AUTHORS and
  SKIP_WRITE_GIT_CHANGELOG environment variable for packagers.

Change-Id: I153c26f0d6876729dfb95eed2ab031c2aaa890d5
Signed-off-by: Vasudev Kamath <kamathvasudev@gmail.com>