FAQ page (wiki and documentation)

[ricalessandri]

I’ve started the PySAGES wiki: https://github.com/SSAGESLabs/PySAGES/wiki
With our FAQ (and first question):
https://github.com/SSAGESLabs/PySAGES/wiki/Frequently-Asked-Questions
Feel free to add more.

As discussed with @pabloferz, we may want to have this section (or a link to the wiki?) in our documentation, too.