add mention of HADOOP_CLASSPATH
add mention of HADOOP_CLASSPATH

costin/elasticsearch-hadoop,pranavraman/elasticsearch-hadoop,takezoe/elasticsearch-hadoop,aie108/elasticsearch-hadoop,girirajsharma/elasticsearch-hadoop,kai5263499/elasticsearch-hadoop,yonglehou/elasticsearch-hadoop,xjrk58/elasticsearch-hadoop,elastic/elasticsearch-hadoop,holdenk/elasticsearch-hadoop,cgvarela/elasticsearch-hadoop,sarwarbhuiyan/elasticsearch-hadoop,huangll/elasticsearch-hadoop,jasontedor/elasticsearch-hadoop,elastic/elasticsearch-hadoop,trifork/elasticsearch-hadoop,nfouka/elasticsearch-hadoop,Gavin-Yang/elasticsearch-hadoop,samkohli/elasticsearch-hadoop,lgscofield/elasticsearch-hadoop,puneetjaiswal/elasticsearch-hadoop