add example usage for shard_id in context feature [ticket:2031]
add example usage for shard_id in context feature [ticket:2031]

alex/sqlalchemy,WinterNis/sqlalchemy,davidfraser/sqlalchemy,j5int/sqlalchemy,olemis/sqlalchemy,bdupharm/sqlalchemy,robin900/sqlalchemy,sandan/sqlalchemy,ThiefMaster/sqlalchemy,276361270/sqlalchemy,graingert/sqlalchemy,epa/sqlalchemy,Cito/sqlalchemy,davidjb/sqlalchemy,pdufour/sqlalchemy,elelianghh/sqlalchemy,WinterNis/sqlalchemy,j5int/sqlalchemy,halfcrazy/sqlalchemy,ThiefMaster/sqlalchemy,itkovian/sqlalchemy,EvaSDK/sqlalchemy,Cito/sqlalchemy,monetate/sqlalchemy,276361270/sqlalchemy,wfxiang08/sqlalchemy,inspirehep/sqlalchemy,hsum/sqlalchemy,pdufour/sqlalchemy,Akrog/sqlalchemy,alex/sqlalchemy,graingert/sqlalchemy,brianv0/sqlalchemy,dstufft/sqlalchemy,hsum/sqlalchemy,dstufft/sqlalchemy,monetate/sqlalchemy,zzzeek/sqlalchemy,bootandy/sqlalchemy,wujuguang/sqlalchemy,Akrog/sqlalchemy,sandan/sqlalchemy,wujuguang/sqlalchemy,itkovian/sqlalchemy,inspirehep/sqlalchemy,Cito/sqlalchemy,epa/sqlalchemy,alex/sqlalchemy,olemis/sqlalchemy,sqlalchemy/sqlalchemy,bootandy/sqlalchemy,davidjb/sqlalchemy,halfcrazy/sqlalchemy,wfxiang08/sqlalchemy,EvaSDK/sqlalchemy,davidfraser/sqlalchemy,brianv0/sqlalchemy,bdupharm/sqlalchemy,robin900/sqlalchemy,elelianghh/sqlalchemy