README.rst: tweaks

Change-Id: Iadb60bd8422734a0eb252d30accd513a49032fd3
Make source configurable when using --coverage

Change-Id: If5cd9e517d66322027408a59fd99015c91181db8
Closes-bug: #1310008
Fixed a typo in the documentation.

Found using: https://github.com/intgr/topy

Change-Id: I15d9469e60ea74aacba03b5ad0e50b4a87661892
Merge "README.rst: tweaks"
Merge "Make source configurable when using --coverage"
Merge "Fixed a typo in the documentation."
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>