[docu] improve course installation description

Here's the improvement that was suggested to improve the course installation documentation. Closes #30 (closed).
During my implementation, it turned out that there is a different procedure necessary depending on whether you want to install everything or just the course (into an existing DuMu^x installation). I described both and tried to keep it as clear and short as possible without missing the core of the matter.

Please let me know where we should enhance this or if this documentation is clear now.