Added checkboxes to PULL_REQUEST_TEMPLATE.md
Added checkboxes to PULL_REQUEST_TEMPLATE.md

See https://github.com/blog/1375-task-lists-in-gfm-issues-pulls-comments for more details.

zaneli/playframework,playframework/playframework,aradchykov/playframework,marcospereira/playframework,wsargent/playframework,benmccann/playframework,wsargent/playframework,aradchykov/playframework,hagl/playframework,marcospereira/playframework,playframework/playframework,ktoso/playframework,rajeshpg/playframework,wsargent/playframework,rajeshpg/playframework,zaneli/playframework,marcospereira/playframework,Shenker93/playframework,mkurz/playframework,marcospereira/playframework,playframework/playframework,rajeshpg/playframework,wegtam/playframework,Shruti9520/playframework,zaneli/playframework,richdougherty/playframework,aradchykov/playframework,wegtam/playframework,benmccann/playframework,wegtam/playframework,mkurz/playframework,Shruti9520/playframework,mkurz/playframework,wsargent/playframework,ktoso/playframework,hagl/playframework,richdougherty/playframework,benmccann/playframework,rajeshpg/playframework,hagl/playframework,Shruti9520/playframework,aradchykov/playframework,benmccann/playframework,ktoso/playframework,Shenker93/playframework,ktoso/playframework,richdougherty/playframework,hagl/playframework,richdougherty/playframework,zaneli/playframework,Shenker93/playframework,Shenker93/playframework,mkurz/playframework,Shruti9520/playframework,wegtam/playframework