README: Add OR grouping controller code example to
README: Add OR grouping controller code example to

help clarify how it works and help people try it out.

[skip ci]

guilleva/ransack,mariorcardoso/ransack,reinteractive/ransack,outstand/ransack,roman-franko/ransack,nagyt234/ransack,AndrewSwerlick/ransack,Mario245/ransack,activerecord-hackery/ransack,johan--/ransack,vaski/ransack,amboxer21/ransack,avit/ransack,laserlemon/ransack,martndemus/ransack,Eric-Guo/ransack