Add documentation about Q format.
Add documentation about Q format.

larrybradley/astropy,lpsinger/astropy,astropy/astropy,pllim/astropy,lpsinger/astropy,larrybradley/astropy,mhvk/astropy,lpsinger/astropy,lpsinger/astropy,mhvk/astropy,astropy/astropy,pllim/astropy,astropy/astropy,mhvk/astropy,mhvk/astropy,larrybradley/astropy,larrybradley/astropy,pllim/astropy,lpsinger/astropy,mhvk/astropy,larrybradley/astropy,pllim/astropy,astropy/astropy,pllim/astropy,astropy/astropy