[skip ci] Add issues guidelines to README
[skip ci] Add issues guidelines to README

to hopefully reduce the number of issues opened without actionable
information and the "+1" comments :trollface:

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