bugref:8524: Additions/linux: play nicely with distribution-installed Additions
Improve the documentation for packaging and minor installer change.
Add a README file for Linux distribution Additions packagers encouraging them
to track stable versions of the Additions without sticking to one major
version in one distribution release. Update the message when the Additions
installer detects another version installed not to always recommend removing
distribution Additions. Force a version update notification after an attempt
to install the Additions.
