Document build system better

The build system is confusing right now. There seem to be at least two ways to build the project:

• qmake
• cmake

The documentation states:
To run: qmake-qt5 && make && ./Babe

This does indeed work, but the resulting artifacts (binaries, image resources, etc.) for oneself, which is bad for all of the distributions trying to package your software.

In order to get your software easily packaged in the major Linux distributions you should provide a simple way to build and install your software and document it appropriately. Usually this looks something like this for KDE projects:

cmake -d build
make
make install