Setup publish settings via build properties

Background
=========

When checking out the repository for the first time it is not possible to immediately build the project. I aimed to solve that, but as the path to the KSP folder differs from machine to machine it seems not to be possible to just "checkout and build".
However, I *tried* to reduce the number of steps neccessary to build for the first time.

What is included?
==============

One thing I could improve was the effort to setup your own KSP directory for publishing (copying files to the KSP directory). Now it is essentially only neccessary to set one variable, instead of replacing multiple lines in the PostBuild-script.

I also tried to summarize the steps neccessary for building in the README-file.

Changes
-----------
* KSP-dir in PostBuild is now specified by single build property
* publishing to KSP-Dir is now optional
* README was updated accordingly

Best regards
Jan