Add missing 'instances' variable to plugin doc
Fix bug where an empty config from the main class would let config merge fail
instructions to setup and run acceptance tests

Although these are dedicated to Ubuntu 14.04 they can be useful hints to
other operating system users.

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #142 from dachary/wip-contributing

instructions to setup and run acceptance tests
Touch up contributing file a bit