add contributors link and change credits to maintainers
add contributors link and change credits to maintainers

estum/validates_timeliness,StatProSA/sp-validates_timeliness,spjsschl/validates_timeliness,munirent/validates_timeliness,johnnyshields/validates_timeliness,kirkgraham/validates_timeliness,nishaya/validates_timeliness,monkbroc/validates_timeliness,johncarney/validates_timeliness,adzap/validates_timeliness,appfolio/validates_timeliness,yez/validates_timeliness