Update VERSION comment with instructions

Instructions about separate source and Debian package versioning.
Removed of libsokoengineqt package.

This library exists solely to be able to link tests against it. Otherwise,
it would be integral part of bosp_app. Instead of creating shared,
installable library, we build it as static library and link BoSp app and
tests with it. Library itself is never installed.
Cleaning up
Merge branch 'feature/reorganize_debian_package' into development

Removed libsokoengineqt package. This library existed solely to be
able to link test suite against it. It is now built as static library
and both, BoSp app and tests are linked against it. Package for that
library is no longer needed.