README: further improvements to OR grouping section
README: further improvements to OR grouping section

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