Add documentation for Riemann sink.
Add documentation for Riemann sink.

Signed-off-by: Vishnu kannan <8c9a7175bd62235d51889a4697184181a7bcfb30@google.com>

afein/heapster,liubog2008/heapster,titilambert/heapster,adfin/statster,saiwl/heapster,aegixx/heapster,piosz/heapster,jackzampolin/heapster,mwielgus/heapster,AlmogBaku/heapster,piosz/heapster,krzyzacy/heapster,johanneswuerbach/heapster,johanneswuerbach/heapster,andyxning/heapster,taimir/heapster,shilpamayanna/heapster,vicki-c/heapster,wulonghui/heapster,shilpamayanna/heapster,a-robinson/heapster,krzyzacy/heapster,a-robinson/heapster,AlmogBaku/heapster,skytap/heapster,mwringe/heapster,Stackdriver/heapster,bluebreezecf/heapster,aegixx/heapster,adfin/statster,saiwl/heapster,wulonghui/heapster,mwielgus/heapster,Huawei-PaaS/heapster,taimir/heapster,mwringe/heapster,whuwxl/heapster,bruceauyeung/heapster,liubog2008/heapster,burmanm/heapster,vicki-c/heapster,titilambert/heapster,burmanm/heapster,skytap/heapster,whuwxl/heapster,luxas/heapster,Stackdriver/heapster,afein/heapster,huangyuqi/heapster,jackzampolin/heapster,andyxning/heapster,Huawei-PaaS/heapster,luxas/heapster,huangyuqi/heapster,bruceauyeung/heapster,bluebreezecf/heapster