[SPARK-17736][DOCUMENTATION][SPARKR] Update R README for rmarkdown,…
[SPARK-17736][DOCUMENTATION][SPARKR] Update R README for rmarkdown,…

## What changes were proposed in this pull request?

To build R docs (which are built when R tests are run), users need to install pandoc and rmarkdown. This was done for Jenkins in ~~[SPARK-17420](https://issues.apache.org/jira/browse/SPARK-17420)~~

… pandoc]

Author: Jagadeesan <as2@us.ibm.com>

Closes #15309 from jagadeesanas2/SPARK-17736.

alunarbeach/spark,Panos-Bletsos/spark-cost-model-optimizer,dbtsai/spark,dbtsai/spark,shaneknapp/spark,rikima/spark,UndeadBaneGitHub/spark,jrshust/spark,srowen/spark,skonto/spark,BryanCutler/spark,eyalfa/spark,bdrillard/spark,saltstar/spark,SnappyDataInc/spark,jkbradley/spark,kimoonkim/spark,stanzhai/spark,guoxiaolongzte/spark,sureshthalamati/spark,narahari92/spark,SnappyDataInc/spark,saturday-shi/spark,sahilTakiar/spark,zuotingbing/spark,zhouyejoe/spark,zero323/spark,hvanhovell/spark,ericvandenbergfb/spark,maropu/spark,stanzhai/spark,mike0sv/spark,rekhajoshm/spark,ptkool/spark,andrewor14/spark,sachintyagi22/spark,zuotingbing/spark,joseph-torres/spark,saltstar/spark,wzhfy/spark,map222/spark,byakuinss/spark,maropu/spark,hhbyyh/spark,wzhfy/spark,gengliangwang/spark,saturday-shi/spark,zuotingbing/spark,lxsmnv/spark,jlopezmalla/spark,srowen/spark,BryanCutler/spark,ahnqirage/spark,tengpeng/spark,ueshin/apache-spark,eyalfa/spark,xuanyuanking/spark,andrewor14/spark,bravo-zhang/spark,cin/spark,sahilTakiar/spark,adrian-ionescu/apache-spark,lvdongr/spark,nilsgrabbert/spark,kimoonkim/spark,huang1900/spark,wzhfy/spark,zero323/spark,byakuinss/spark,lvdongr/spark,darionyaphet/spark,rednaxelafx/apache-spark,brad-kaiser/spark,map222/spark,srowen/spark,milliman/spark,witgo/spark,ddna1021/spark,panfengfeng/spark,Panos-Bletsos/spark-cost-model-optimizer,patrick-nicholson/spark,someorz/spark,rednaxelafx/apache-spark,WeichenXu123/spark,techaddict/spark,someorz/spark,rednaxelafx/apache-spark,jlopezmalla/spark,saturday-shi/spark,ConeyLiu/spark,zzcclp/spark,highfei2011/spark,ajaysaini725/spark,BryanCutler/spark,techaddict/spark,JerryLead/spark,narahari92/spark,jlopezmalla/spark,Aegeaner/spark,loneknightpy/spark,huang1900/spark,rikima/spark,1haodian/spark,hvanhovell/spark,WindCanDie/spark,cloud-fan/spark,darionyaphet/spark,alunarbeach/spark,MLnick/spark,apache-spark-on-k8s/spark,tejasapatil/spark,mdespriee/spark,ahnqirage/spark,xuanyuanking/spark,aosagie/spark,debugger87/spark,bravo-zhang/spark,alunarbeach/spark,yanboliang/spark,SHASHANKB/spark,ericvandenbergfb/spark,apache/spark,narahari92/spark,esi-mineset/spark,patrick-nicholson/spark,zhouyejoe/spark,zhouyejoe/spark,rikima/spark,skonto/spark,szhem/spark,setjet/spark,minixalpha/spark,rezasafi/spark,michalsenkyr/spark,janewangfb/spark,gengliangwang/spark,tengpeng/spark,milliman/spark,vinodkc/spark,ioana-delaney/spark,JerryLead/spark,spark-test/spark,hhbyyh/spark,nilsgrabbert/spark,holdenk/spark,mdespriee/spark,someorz/spark,alunarbeach/spark,lxsmnv/spark,dhruve/spark,nchammas/spark,jianran/spark,WeichenXu123/spark,MLnick/spark,mdespriee/spark,nchammas/spark,jianran/spark,big-pegasus/spark,brad-kaiser/spark,sachintyagi22/spark,dongjoon-hyun/spark,1haodian/spark,liyichao/spark,liutang123/spark,minixalpha/spark,narahari92/spark,joseph-torres/spark,cin/spark,rekhajoshm/spark,saltstar/spark,ueshin/apache-spark,ahnqirage/spark,guoxiaolongzte/spark,kiszk/spark,akopich/spark,tejasapatil/spark,liyichao/spark,joseph-torres/spark,gengliangwang/spark,skonto/spark,map222/spark,rekhajoshm/spark,ericvandenbergfb/spark,someorz/spark,SnappyDataInc/spark,cin/spark,adrian-ionescu/apache-spark,nilsgrabbert/spark,rikima/spark,hvanhovell/spark,esi-mineset/spark,ptkool/spark,shaneknapp/spark,ueshin/apache-spark,big-pegasus/spark,Panos-Bletsos/spark-cost-model-optimizer,maropu/spark,sachintyagi22/spark,nilsgrabbert/spark,goldmedal/spark,wangmiao1981/spark,map222/spark,esi-mineset/spark,dhruve/spark,wzhfy/spark,metamx/spark,ibm-research-ireland/sparkoscope,lvdongr/spark,icexelloss/spark,bravo-zhang/spark,huang1900/spark,ahnqirage/spark,nchammas/spark,shuangshuangwang/spark,lvdongr/spark,apache-spark-on-k8s/spark,Aegeaner/spark,SnappyDataInc/spark,witgo/spark,rezasafi/spark,markhamstra/spark,huang1900/spark,ron8hu/spark,ioana-delaney/spark,ptkool/spark,bOOm-X/spark,ron8hu/spark,wzhfy/spark,lxsmnv/spark,gengliangwang/spark,SHASHANKB/spark,kissmetrics/spark,map222/spark,bOOm-X/spark,JerryLead/spark,shaneknapp/spark,minixalpha/spark,1haodian/spark,SnappyDataInc/spark,UndeadBaneGitHub/spark,matthewfranglen/spark,adrian-ionescu/apache-spark,michalsenkyr/spark,Panos-Bletsos/spark-cost-model-optimizer,taroplus/spark,yanboliang/spark,joseph-torres/spark,rezasafi/spark,highfei2011/spark,esi-mineset/spark,ConeyLiu/spark,holdenk/spark,tejasapatil/spark,kimoonkim/spark,rikima/spark,jiangxb1987/spark,darionyaphet/spark,ajaysaini725/spark,bOOm-X/spark,WindCanDie/spark,cloud-fan/spark,stanzhai/spark,nchammas/spark,rednaxelafx/apache-spark,bdrillard/spark,szhem/spark,map222/spark,janewangfb/spark,kissmetrics/spark,panfengfeng/spark,map222/spark,andrewor14/spark,bdrillard/spark,metamx/spark,MLnick/spark,mzl9039/spark,setjet/spark,SHASHANKB/spark,joseph-torres/spark,jiangxb1987/spark,panfengfeng/spark,mdespriee/spark,aosagie/spark,brad-kaiser/spark,aosagie/spark,michalsenkyr/spark,JoshRosen/spark,WeichenXu123/spark,WindCanDie/spark,dongjoon-hyun/spark,BryanCutler/spark,liutang123/spark,aokolnychyi/spark,ajaysaini725/spark,li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