Added plugin installation instructions
Added plugin installation instructions

Original commit: elastic/x-pack-elasticsearch@55b7cd933149d5118ef1212949ab3ab32f49bdb2

uschindler/elasticsearch,uschindler/elasticsearch,scorpionvicky/elasticsearch,nknize/elasticsearch,GlenRSmith/elasticsearch,gingerwizard/elasticsearch,strapdata/elassandra,gingerwizard/elasticsearch,HonzaKral/elasticsearch,GlenRSmith/elasticsearch,coding0011/elasticsearch,nknize/elasticsearch,gfyoung/elasticsearch,strapdata/elassandra,robin13/elasticsearch,robin13/elasticsearch,nknize/elasticsearch,vroyer/elassandra,gingerwizard/elasticsearch,GlenRSmith/elasticsearch,GlenRSmith/elasticsearch,GlenRSmith/elasticsearch,robin13/elasticsearch,strapdata/elassandra,scorpionvicky/elasticsearch,robin13/elasticsearch,vroyer/elassandra,uschindler/elasticsearch,robin13/elasticsearch,gfyoung/elasticsearch,HonzaKral/elasticsearch,gingerwizard/elasticsearch,gingerwizard/elasticsearch,uschindler/elasticsearch,nknize/elasticsearch,vroyer/elassandra,gingerwizard/elasticsearch,coding0011/elasticsearch,scorpionvicky/elasticsearch,gfyoung/elasticsearch,nknize/elasticsearch,gfyoung/elasticsearch,strapdata/elassandra,scorpionvicky/elasticsearch,coding0011/elasticsearch,strapdata/elassandra,uschindler/elasticsearch,HonzaKral/elasticsearch,HonzaKral/elasticsearch,coding0011/elasticsearch,scorpionvicky/elasticsearch,gingerwizard/elasticsearch,gfyoung/elasticsearch,coding0011/elasticsearch