DOC : Fix virtual env doc

[RobinCamarasa]

Good evening,

I have been a beta tester of SimpleElastix for a workshop organized by Emanoel Sabidussi with the help Stefan Klein. I found the Compiling On Linux documentation a bit tricky about virtual environment. Indeed, if you follow the documentation blindely copy pasting commands as the following "sudo python Packaging/setup.py install" will install SimpleElastix in your default python2 default environment even if your virtual environment is loaded. That's why I proposed a fix on the documentation with two options one with virtual environment and one without. A more global remark I think that it 's better practice to make a warning to explain that sudo rights are required rather than adding "sudo" at the beginning of each command.
Furthermore, my fix is clearly not perfect, it is the first time that I fork a repo and contribute to library so I might have done something wrong. But I hope that what I did can help in a way.

Best
Robin

[kaspermarstal]

Hi @RobinCamarasa thank you very much for your contribution!