add doc about writing a topology (#749)
add doc about writing a topology (#749)

* add doc about writing a topology
moomou/heron,moomou/heron,zhangzhonglai/heron,tomncooper/heron,lukess/heron,cs564/heron,mycFelix/heron,cs564/heron,wangli1426/heron,cs564/heron,cs564/heron,cs564/heron,ashvina/heron,twitter/heron,wangli1426/heron,tomncooper/heron,lewiskan/heron,lewiskan/heron,tomncooper/heron,cliffyg/heron,lukess/heron,lewiskan/heron,objmagic/heron,objmagic/heron,ashvina/heron,cliffyg/heron,mycFelix/heron,huijunwu/heron,zhangzhonglai/heron,mycFelix/heron,lucperkins/heron,cs564/heron,lukess/heron,ashvina/heron,billonahill/heron,cs564/heron,billonahill/heron,streamlio/heron,tomncooper/heron,huijunwu/heron,moomou/heron,nlu90/heron,lewiskan/heron,lukess/heron,zhangzhonglai/heron,mycFelix/heron,huijunwu/heron,srkukarni/heron,streamlio/heron,mycFelix/heron,lukess/heron,moomou/heron,mycFelix/heron,lucperkins/heron,cs564/heron,srkukarni/heron,wangli1426/heron,tomncooper/heron,mycFelix/heron,moomou/heron,moomou/heron,streamlio/heron,moomou/heron,srkukarni/heron,streamlio/heron,huijunwu/heron,lukess/heron,cliffyg/heron,lucperkins/heron,billonahill/heron,billonahill/heron,lucperkins/heron,ashvina/heron,srkukarni/heron,streamlio/heron,objmagic/heron,lewiskan/heron,twitter/heron,srkukarni/heron,twitter/heron,lewiskan/heron,zhangzhonglai/heron,zhangzhonglai/heron,cs564/heron,zhangzhonglai/heron,cliffyg/heron,moomou/heron,tomncooper/heron,ashvina/heron,tomncooper/heron,cliffyg/heron,lucperkins/heron,ashvina/heron,lukess/heron,wangli1426/heron,wangli1426/heron,twitter/heron,objmagic/heron,zhangzhonglai/heron,wangli1426/heron,nlu90/heron,billonahill/heron,ashvina/heron,ashvina/heron,cliffyg/heron,lewiskan/heron,srkukarni/heron,objmagic/heron,objmagic/heron,lewiskan/heron,nlu90/heron,wangli1426/heron,twitter/heron,mycFelix/heron,zhangzhonglai/heron,srkukarni/heron,twitter/heron,ashvina/heron,nlu90/heron,tomncooper/heron,cliffyg/heron,nlu90/heron,wangli1426/heron,lucperkins/heron,tomncooper/heron,moomou/heron,streamlio/heron,billonahill/heron,lucperkins/heron,cliffyg/heron,cliffyg/heron,huijunwu/heron,streamlio/heron,lukess/heron,billonahill/heron,nlu90/heron,huijunwu/heron,billonahill/heron,lukess/heron,streamlio/heron,srkukarni/heron,zhangzhonglai/heron,nlu90/heron,twitter/heron,wangli1426/heron,mycFelix/heron,huijunwu/heron,billonahill/heron,objmagic/heron,lewiskan/heron,nlu90/heron,twitter/heron,objmagic/heron,nlu90/heron,huijunwu/heron,lucperkins/heron,streamlio/heron,objmagic/heron