Commonly used shells section; elaborate on POSIX

[cochrand]

What's the point of listing all these shells? Am I supposed to be able to access each of them? Which are relevant to this material? I presume that bash (and perhaps csh/tcsh) is what we're really supposed to be concentrating on, so this material should be presented in a way that makes that clear. Also, what is POSIX and why do I care???

[yarikoptic]

as for the "point", it is in the section title ... and their relevance to existing neuroimaging projects. I find frequently people talking about all the shells without realizing that they are very much different. And many well established neuroimaging projects use different shells.

POSIX -- indeed we should add a reference and reason for knowing about it existence since it is a set of standards, and without adhering to commonly accepted standards we would get nowhere, neuroimaging including. Adding to the issue title