Rewrote Linux part of Parallel Install Instructions, please review

Hi documenters,

whoever wants to install a new LibreOffice version in parallel in order
to test it or to try out new features:

I rewrote the intstructions for Linux to make them more
comprehensible[1].

Feedback welcome (I'm not a native English speaker).

Please be aware that installing developmental, beta or RC releases might
screw up your system. Therefore, it is recommended to install them on
testing machines only.

Nino
[1] http://wiki.documentfoundation.org/Installing_in_parallel