readme: update ETL description (#8369)
readme: update ETL description (#8369)

Related comment: https://github.com/pingcap/docs/pull/677#issuecomment-430252483
XuHuaiyu/tidb,ngaut/tidb,qiuyesuifeng/tidb,qiuyesuifeng/tidb,EvilMcJerkface/tidb,coocood/tidb,kyle-wang/tidb,pingcap/tidb,kyle-wang/tidb,kyle-wang/tidb,cosmtrek/tidb,kyle-wang/tidb,kyle-wang/tidb,XuHuaiyu/tidb,pingcap/tidb,coocood/tidb,ming-relax/tidb,EvilMcJerkface/tidb,ngaut/tidb,pingcap/tidb,mccxj/tidb,pingcap/tidb,kyle-wang/tidb,cosmtrek/tidb,pingcap/tidb,c4pt0r/tidb,ngaut/tidb,mccxj/tidb,c4pt0r/tidb,ngaut/tidb,c4pt0r/tidb,c4pt0r/tidb,c4pt0r/tidb,ngaut/tidb,ngaut/tidb,Cofyc/tidb,pingcap/tidb,c4pt0r/tidb,ming-relax/tidb,Cofyc/tidb