Updated the INSTALL file. Added some steps to follow if Krusader is already installed

The INSTALL file was updated; for example KDE Plasma 5 is not required to run Krusader, as it's written in
    https://docs.kde.org/trunk5/en/extragear-utils/krusader/faq.html#idm45666073100176
    https://docs.kde.org/trunk5/en/extragear-utils/krusader/installation.html

Added some steps to follow if Krusader is already installed.

Differential Revision: https://phabricator.kde.org/D12600