Add Yahoo's blog post about Sherpa to USERS.md
Add Yahoo's blog post about Sherpa to USERS.md

Summary: As title

Test Plan: ran unit tests

Reviewers: sdong, yhchiang, rven

Reviewed By: rven

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D39765

kaschaeffer/rocksdb,Applied-Duality/rocksdb,luckywhu/rocksdb,ryneli/rocksdb,vmx/rocksdb,JackLian/rocksdb,siddhartharay007/rocksdb,temicai/rocksdb,bbiao/rocksdb,wskplho/rocksdb,Andymic/rocksdb,ylong/rocksdb,facebook/rocksdb,facebook/rocksdb,SunguckLee/RocksDB,makelivedotnet/rocksdb,skunkwerks/rocksdb,tizzybec/rocksdb,wat-ze-hex/rocksdb,wlqGit/rocksdb,siddhartharay007/rocksdb,tizzybec/rocksdb,siddhartharay007/rocksdb,anagav/rocksdb,ryneli/rocksdb,luckywhu/rocksdb,biddyweb/rocksdb,makelivedotnet/rocksdb,IMCG/RcoksDB,rDSN-Projects/rocksdb.replicated,facebook/rocksdb,Applied-Duality/rocksdb,JohnPJenkins/rocksdb,Andymic/rocksdb,wenduo/rocksdb,zhangpng/rocksdb,OverlordQ/rocksdb,temicai/rocksdb,JoeWoo/rocksdb,vmx/rocksdb,wenduo/rocksdb,Andymic/rocksdb,vmx/rocksdb,vashstorm/rocksdb,JackLian/rocksdb,hobinyoon/rocksdb,geraldoandradee/rocksdb,SunguckLee/RocksDB,RyanTech/rocksdb,bbiao/rocksdb,norton/rocksdb,mbarbon/rocksdb,JackLian/rocksdb,tizzybec/rocksdb,Applied-Duality/rocksdb,ylong/rocksdb,rDSN-Projects/rocksdb.replicated,wlqGit/rocksdb,caijieming-baidu/rocksdb,norton/rocksdb,jalexanderqed/rocksdb,SunguckLee/RocksDB,hobinyoon/rocksdb,rDSN-Projects/rocksdb.replicated,hobinyoon/rocksdb,vashstorm/rocksdb,vmx/rocksdb,luckywhu/rocksdb,tsheasha/rocksdb,hobinyoon/rocksdb,mbarbon/rocksdb,norton/rocksdb,geraldoandradee/rocksdb,dkorolev/rocksdb,vmx/rocksdb,skunkwerks/rocksdb,makelivedotnet/rocksdb,alihalabyah/rocksdb,ylong/rocksdb,vashstorm/rocksdb,dkorolev/rocksdb,sorphi/rocksdb,biddyweb/rocksdb,tizzybec/rocksdb,mbarbon/rocksdb,anagav/rocksdb,wenduo/rocksdb,wlqGit/rocksdb,Andymic/rocksdb,lgscofield/rocksdb,jalexanderqed/rocksdb,vmx/rocksdb,JohnPJenkins/rocksdb,bbiao/rocksdb,makelivedotnet/rocksdb,JoeWoo/rocksdb,ryneli/rocksdb,JohnPJenkins/rocksdb,luckywhu/rocksdb,dkorolev/rocksdb,temicai/rocksdb,wat-ze-hex/rocksdb,wat-ze-hex/rocksdb,luckywhu/rocksdb,caijieming-baidu/rocksdb,mbarbon/rocksdb,SunguckLee/RocksDB,biddyweb/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,facebook/rocksdb,JohnPJenkins/rocksdb,flabby/rocksdb,jalexanderqed/rocksdb,sorphi/rocksdb,Andymic/rocksdb,bbiao/rocksdb,OverlordQ/rocksdb,ylong/rocksdb,geraldoandradee/rocksdb,zhangpng/rocksdb,wat-ze-hex/rocksdb,sorphi/rocksdb,tsheasha/rocksdb,caijieming-baidu/rocksdb,sorphi/rocksdb,luckywhu/rocksdb,Vaisman/rocksdb,rDSN-Projects/rocksdb.replicated,siddhartharay007/rocksdb,IMCG/RcoksDB,SunguckLee/RocksDB,facebook/rocksdb,skunkwerks/rocksdb,JoeWoo/rocksdb,tizzybec/rocksdb,kaschaeffer/rocksdb,Vaisman/rocksdb,mbarbon/rocksdb,fengshao0907/rocksdb,fengshao0907/rocksdb,JackLian/rocksdb,biddyweb/rocksdb,Vaisman/rocksdb,norton/rocksdb,fengshao0907/rocksdb,JoeWoo/rocksdb,tsheasha/rocksdb,lgscofield/rocksdb,Vaisman/rocksdb,rDSN-Projects/rocksdb.replicated,Applied-Duality/rocksdb,luckywhu/rocksdb,tsheasha/rocksdb,skunkwerks/rocksdb,flabby/rocksdb,IMCG/RcoksDB,ryneli/rocksdb,zhangpng/rocksdb,vashstorm/rocksdb,wlqGit/rocksdb,temicai/rocksdb,wskplho/rocksdb,IMCG/RcoksDB,alihalabyah/rocksdb,wskplho/rocksdb,hobinyoon/rocksdb,wlqGit/rocksdb,ryneli/rocksdb,lgscofield/rocksdb,ylong/rocksdb,jalexanderqed/rocksdb,sorphi/rocksdb,zhangpng/rocksdb,temicai/rocksdb,fengshao0907/rocksdb,vmx/rocksdb,mbarbon/rocksdb,flabby/rocksdb,Applied-Duality/rocksdb,sorphi/rocksdb,wskplho/rocksdb,norton/rocksdb,tsheasha/rocksdb,fengshao0907/rocksdb,Applied-Duality/rocksdb,OverlordQ/rocksdb,biddyweb/rocksdb,geraldoandradee/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,kaschaeffer/rocksdb,geraldoandradee/rocksdb,alihalabyah/rocksdb,caijieming-baidu/rocksdb,anagav/rocksdb,jalexanderqed/rocksdb,tsheasha/rocksdb,flabby/rocksdb,anagav/rocksdb,facebook/rocksdb,tizzybec/rocksdb,dkorolev/rocksdb,norton/rocksdb,kaschaeffer/rocksdb,tsheasha/rocksdb,lgscofield/rocksdb,skunkwerks/rocksdb,kaschaeffer/rocksdb,skunkwerks/rocksdb,wenduo/rocksdb,caijieming-baidu/rocksdb,temicai/rocksdb,OverlordQ/rocksdb,JohnPJenkins/rocksdb,OverlordQ/rocksdb,JohnPJenkins/rocksdb,alihalabyah/rocksdb,RyanTech/rocksdb,makelivedotnet/rocksdb,zhangpng/rocksdb,jalexanderqed/rocksdb,vashstorm/rocksdb,OverlordQ/rocksdb,IMCG/RcoksDB,JackLian/rocksdb,anagav/rocksdb,RyanTech/rocksdb,wenduo/rocksdb,anagav/rocksdb,Andymic/rocksdb,norton/rocksdb,ylong/rocksdb,hobinyoon/rocksdb,geraldoandradee/rocksdb,sorphi/rocksdb,RyanTech/rocksdb,bbiao/rocksdb,alihalabyah/rocksdb,fengshao0907/rocksdb,alihalabyah/rocksdb,facebook/rocksdb,rDSN-Projects/rocksdb.replicated,JoeWoo/rocksdb,siddhartharay007/rocksdb,Vaisman/rocksdb,ryneli/rocksdb,JoeWoo/rocksdb,skunkwerks/rocksdb,Vaisman/rocksdb,Andymic/rocksdb,wlqGit/rocksdb,kaschaeffer/rocksdb,dkorolev/rocksdb,biddyweb/rocksdb,ylong/rocksdb,caijieming-baidu/rocksdb,siddhartharay007/rocksdb,wat-ze-hex/rocksdb,RyanTech/rocksdb,JohnPJenkins/rocksdb,siddhartharay007/rocksdb,lgscofield/rocksdb,kaschaeffer/rocksdb,ryneli/rocksdb,caijieming-baidu/rocksdb,rDSN-Projects/rocksdb.replicated,norton/rocksdb,wat-ze-hex/rocksdb,vmx/rocksdb,IMCG/RcoksDB,SunguckLee/RocksDB,zhangpng/rocksdb,Vaisman/rocksdb,flabby/rocksdb,flabby/rocksdb,jalexanderqed/rocksdb,RyanTech/rocksdb,makelivedotnet/rocksdb,geraldoandradee/rocksdb,lgscofield/rocksdb,wenduo/rocksdb,wat-ze-hex/rocksdb,IMCG/RcoksDB,bbiao/rocksdb,wskplho/rocksdb,anagav/rocksdb,zhangpng/rocksdb,dkorolev/rocksdb,JackLian/rocksdb,hobinyoon/rocksdb,tizzybec/rocksdb,wskplho/rocksdb,wat-ze-hex/rocksdb,wenduo/rocksdb,lgscofield/rocksdb,temicai/rocksdb,wlqGit/rocksdb,wenduo/rocksdb,Applied-Duality/rocksdb,JoeWoo/rocksdb,vashstorm/rocksdb,RyanTech/rocksdb,dkorolev/rocksdb,JackLian/rocksdb,wskplho/rocksdb,biddyweb/rocksdb,mbarbon/rocksdb,tsheasha/rocksdb,hobinyoon/rocksdb,fengshao0907/rocksdb,bbiao/rocksdb,facebook/rocksdb,OverlordQ/rocksdb,makelivedotnet/rocksdb,flabby/rocksdb,alihalabyah/rocksdb,vashstorm/rocksdb