Update tuning.md
Update tuning.md

Clarify when serializer is used based on recent user@ mailing list discussion.
aokolnychyi/spark,zuotingbing/spark,cloud-fan/spark,gioenn/xSpark,bdrillard/spark,zzcclp/spark,hvanhovell/spark,bdrillard/spark,zero323/spark,map222/spark,brad-kaiser/spark,vinodkc/spark,byakuinss/spark,esi-mineset/spark,lvdongr/spark,michalsenkyr/spark,aosagie/spark,kimoonkim/spark,techaddict/spark,michalsenkyr/spark,ahnqirage/spark,zero323/spark,ahnqirage/spark,brad-kaiser/spark,bOOm-X/spark,apache-spark-on-k8s/spark,shaneknapp/spark,JoshRosen/spark,holdenk/spark,esi-mineset/spark,liutang123/spark,jiangxb1987/spark,BryanCutler/spark,minixalpha/spark,vinodkc/spark,dbtsai/spark,ericvandenbergfb/spark,BryanCutler/spark,JerryLead/spark,jkbradley/spark,sryza/spark,nilsgrabbert/spark,narahari92/spark,taroplus/spark,ptkool/spark,highfei2011/spark,janewangfb/spark,markhamstra/spark,debugger87/spark,apache-spark-on-k8s/spark,publicRoman/spark,ConeyLiu/spark,taroplus/spark,apache-spark-on-k8s/spark,rednaxelafx/apache-spark,mike0sv/spark,bOOm-X/spark,jlopezmalla/spark,alunarbeach/spark,kiszk/spark,andrewor14/spark,jiangxb1987/spark,tengpeng/spark,yanboliang/spark,loneknightpy/spark,ibm-research-ireland/sparkoscope,mahak/spark,UndeadBaneGitHub/spark,stanzhai/spark,ioana-delaney/spark,cloudera/spark,zero323/spark,shaneknapp/spark,vinodkc/spark,1haodian/spark,Panos-Bletsos/spark-cost-model-optimizer,sahilTakiar/spark,MLnick/spark,minixalpha/spark,gengliangwang/spark,pronix/spark,aokolnychyi/spark,aray/spark,szhem/spark,goldmedal/spark,MLnick/spark,liyichao/spark,pgandhi999/spark,cloudera/spark,andrewor14/spark,WeichenXu123/spark,holdenk/spark,nilsgrabbert/spark,someorz/spark,bOOm-X/spark,yanboliang/spark,kiszk/spark,bravo-zhang/spark,setjet/spark,vinodkc/spark,wangmiao1981/spark,markhamstra/spark,cin/spark,jiangxb1987/spark,rikima/spark,rikima/spark,sahilTakiar/spark,szhem/spark,cloud-fan/spark,publicRoman/spark,jlopezmalla/spark,JerryLead/spark,guoxiaolongzte/spark,sachintyagi22/spark,setjet/spark,big-pegasus/spark,nilsgrabbert/spark,kiszk/spark,ddna1021/spark,srowen/spark,szhem/spark,stanzhai/spark,pronix/spark,BryanCutler/spark,zuotingbing/spark,ibm-research-ireland/sparkoscope,techaddict/spark,liyichao/spark,patrick-nicholson/spark,taroplus/spark,rezasafi/spark,yanboliang/spark,nchammas/spark,vinodkc/spark,janewangfb/spark,JoshRosen/spark,kevinyu98/spark,jkbradley/spark,skonto/spark,zhouyejoe/spark,rekhajoshm/spark,WindCanDie/spark,1haodian/spark,wzhfy/spark,hvanhovell/spark,ueshin/apache-spark,andrewor14/iolap,facaiy/spark,haowu80s/spark,HyukjinKwon/spark,nchammas/spark,saturday-shi/spark,caneGuy/spark,cloud-fan/spark,ConeyLiu/spark,jianran/spark,tengpeng/spark,rezasafi/spark,rekhajoshm/spark,kissmetrics/spark,zzcclp/spark,saturday-shi/spark,dbtsai/spark,kissmetrics/spark,wangyum/spark,Aegeaner/spark,skonto/spark,kissmetrics/spark,1haodian/spark,liutang123/spark,minixalpha/spark,apache/spark,hvanhovell/spark,dhruve/spark,witgo/spark,SnappyDataInc/spark,witgo/spark,Aegeaner/spark,rednaxelafx/apache-spark,mkolod/incubator-spark,lvdongr/spark,icexelloss/spark,shuangshuangwang/spark,patrick-nicholson/spark,rekhajoshm/spark,andrewor14/iolap,skonto/spark,ericvandenbergfb/spark,maropu/spark,minixalpha/spark,apache-spark-on-k8s/spark,zuotingbing/spark,nchammas/spark,sachintyagi22/spark,panfengfeng/spark,kissmetrics/spark,wangyum/spark,gengliangwang/spark,LantaoJin/spark,holdenk/spark,shubhamchopra/spark,spark-test/spark,lxsmnv/spark,map222/spark,ddna1021/spark,JerryLead/spark,adrian-ionescu/apache-spark,SHASHANKB/spark,kiszk/spark,goldmedal/spark,goldmedal/spark,goldmedal/spark,caneGuy/spark,aray/spark,zhouyejoe/spark,kimoonkim/spark,haowu80s/spark,rednaxelafx/apache-spark,mahak/spark,facaiy/spark,sahilTakiar/spark,ddna1021/spark,debugger87/spark,esi-mineset/spark,rikima/spark,nchammas/spark,panfengfeng/spark,apache/spark,UndeadBaneGitHub/spark,loneknightpy/spark,kiszk/spark,ajaysaini725/spark,ConeyLiu/spark,ericvandenbergfb/spark,tengpeng/spark,MLnick/spark,shubhamchopra/spark,vinodkc/spark,darionyaphet/spark,dongjoon-hyun/spark,ptkool/spark,pronix/spark,dhruve/spark,ibm-research-ireland/sparkoscope,tejasapatil/spark,poffuomo/spark,publicRoman/spark,darionyaphet/spark,techaddict/spark,ptkool/spark,aray/spark,zhouyejoe/spark,highfei2011/spark,bdrillard/spark,akopich/spark,andrewor14/spark,SHASHANKB/spark,MLnick/spark,kimoonkim/spark,jiangxb1987/spark,zzcclp/spark,dbtsai/spark,icexelloss/spark,highfei2011/spark,Aegeaner/spark,aray/spark,aokolnychyi/spark,stanzhai/spark,pronix/spark,BryanCutler/spark,ddna1021/spark,byakuinss/spark,andrewor14/iolap,esi-mineset/spark,hvanhovell/spark,rezasafi/spark,gioenn/xSpark,debugger87/spark,brad-kaiser/spark,spark-test/spark,dotunolafunmiloye/spark,joseph-torres/spark,adrian-ionescu/apache-spark,andrewor14/spark,janewangfb/spark,joseph-torres/spark,lvdongr/spark,facaiy/spark,ajaysaini725/spark,rednaxelafx/apache-spark,rekhajoshm/spark,jlopezmalla/spark,joseph-torres/spark,pgandhi999/spark,xflin/spark,milliman/spark,xflin/spark,mdespriee/spark,xflin/spark,saturday-shi/spark,shubhamchopra/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