WIP:  FAQ

WIP to add a FAQ to the documentation, following up on https://github.com/mne-tools/mne-python/issues/1495 and https://github.com/mne-tools/mne-python/issues/1637. 
CC @agramfort @dengemann 


Is it possible to build the docs with Anaconda? I get 

```
$ make html
...
plotting plot_bem_contour_mri.py
//anaconda/lib/python2.7/site-packages/wx-3.0-osx_cocoa/wx/_core.py:16674: ImportWarning: Not importing directory '//anaconda/lib/python2.7/site-packages/wx-3.0-osx_cocoa/wx/locale': missing __init__.py
  import locale
This program needs access to the screen.
Please run with a Framework build of python, and only when you are
logged in on the main display of your Mac.
make: *** [html] Error 1
```
This is not the ideal metric, a Chi^2 is better here to detect model violations as it does not subtract any mean.
But with #1671 all this is already brought to you for free by calling evoked.plot_white

Also printing the baseline means is a bit too much. The loglikelihood function used with cross-validation already quantifies this as a metric.