DOC: Add BSD rationale to FAQ

Due to http://projects.scipy.org/scipy/ticket/648#comment:7, I've been reading about BSD vs GPL, and trying to convince myself to rewrite the GPL code from scratch for inclusion in SciPy.  :)  I couldn't find any rationale in the SciPy docs for *why* the BSD license is used, but eventually found [a post in a mailing list](http://mail.scipy.org/pipermail/scipy-user/2007-January/010602.html) by John Hunter (which others in the thread [requested be included in the SciPy FAQ](http://mail.scipy.org/pipermail/scipy-user/2007-January/010621.html), but I don't think it has).

Also found that nipy has [several FAQ questions about this](http://nipy.sourceforge.net/nipy/stable/faq/licensing.html#why-did-you-choose-bsd), and links to [a copy of the same pitch](http://nipy.sourceforge.net/nipy/stable/faq/johns_bsd_pitch.html#johns-bsd-pitch).  Their questions might be included in SciPy's FAQ, too.

Instead of linking to nipy's copy, it might be best to include the pitch directly in scipy, and then have nipy link to scipy's copy instead?

This is just a proposed wording.  Feel free to reject it and write your own, I just want it mentioned somewhere.  Maybe it would be better at http://www.scipy.org/License_Compatibility instead of here? (though I can't edit that)