doc: update CONTRIBUTING.md, add stand-alone mode section. (#5125)
doc: update CONTRIBUTING.md, add stand-alone mode section. (#5125)


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