Improve the setup description of the README file

[CrsiX]

For some users it might be obvious how to use our test suite, but others might have problems with it. For example, the Windows-specific setup description is just as short as:

This has to be executed in the command line, run as administrator:

$ mklink /J \path\to\src\tests \path\to\repository\tests

Besides the fact that on Windows there are no paths that start with a "drive-less" \ but C:\ or D:\ instead, we need a much better and possible longer description for this. Of course, this would also apply to the *nix/BSD/MacOS part, which is just a ln call without further explanation.

Parts of this issue may be tracked in certain Zulip threads as well.

[AntonKluge]

How it also works and what might be easier for some: right-click on the Projekt -> Open Module Settings -> Modules -> Add Content Root -> Choose the corresponding Folder -> Mark as Tests Source -> Apply.
It's not as elegant but works pretty simple without the command line.