link to CONTRIBUTING.md from pull-requests template
link to CONTRIBUTING.md from pull-requests template

wfxiang08/gh-ost,druud62/gh-ost,groenewold-pythian/gh-ost,shuhaowu/gh-ost,groenewold-pythian/gh-ost,shuhaowu/gh-ost,wfxiang08/gh-ost,druud62/gh-ost