few more docs

Hi,

although I might touch these sources again tomorrow, I thought it's good to open a PR in order to receive early feedback (mostly of crude type about missing pieces/paragraphs). Yet, any feedback is welcome :)

tia, F.
this section needs a big fat warning box: installing with `--user` is pretty evil, since Python packages installed there always win... Even if you change `$PYTHONPATH` to include a path to a newer version of e.g. EasyBuild, the version installed via `--user` will take precedence.

Maybe this section should use `--prefix` instead, partly because `--user` is only available in recent versions of the install tools.