Improve CONTRIBUTING.md documentation. (#4707)
Improve CONTRIBUTING.md documentation. (#4707)


8cbx/tidb,yuanwhy/tidb,mccxj/tidb,cosmtrek/tidb,CodeRushing/tidb,zimulala/tidb,zimulala/tidb,coocood/tidb,ming-relax/tidb,c4pt0r/tidb,wusphinx/tidb,kyle-wang/tidb,yuanwhy/tidb,c4pt0r/tidb,EvilMcJerkface/tidb,coocood/tidb,CodeRushing/tidb,pingcap/tidb,tiancaiamao/tidb,qiuyesuifeng/tidb,Cofyc/tidb,louishust/tidb,cosmtrek/tidb,c4pt0r/tidb,kyle-wang/tidb,Nify/tidb,pingcap/tidb,louishust/tidb,ming-relax/tidb,pingcap/tidb,ngaut/tidb,qiuyesuifeng/tidb,XuHuaiyu/tidb,ngaut/tidb,c4pt0r/tidb,kyle-wang/tidb,c4pt0r/tidb,yangwenmai/tidb,kyle-wang/tidb,pingcap/tidb,c4pt0r/tidb,ngaut/tidb,8cbx/tidb,spongedu/tidb,kyle-wang/tidb,pingcap/tidb,ngaut/tidb,ngaut/tidb,yangwenmai/tidb,wusphinx/tidb,kyle-wang/tidb,tiancaiamao/tidb,Cofyc/tidb,EvilMcJerkface/tidb,pingcap/tidb,ngaut/tidb,mccxj/tidb,XuHuaiyu/tidb,spongedu/tidb,Nify/tidb