README: update status badge to Pre-GA (#4392)
README: update status badge to Pre-GA (#4392)


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