add comment describing how to run, minor bug fix in example
add comment describing how to run, minor bug fix in example


Former-commit-id: c57f9da5778b6921a08f544f70d550d3977c9b73
shuheik/dynet,clab/dynet,xunzhang/dynet,shuheik/dynet,clab/dynet,clab/dynet,chunyang-wen/dynet,shuheik/dynet,danielhers/dynet,chunyang-wen/dynet,danielhers/dynet,shuheik/dynet,shuheik/dynet,xunzhang/dynet,danielhers/dynet,clab/dynet,danielhers/dynet,shuheik/dynet,clab/dynet,clab/cnn,chunyang-wen/dynet,danielhers/dynet,danielhers/dynet,clab/dynet,xunzhang/dynet,clab/dynet,shuheik/dynet,chunyang-wen/dynet,danielhers/dynet,clab/cnn,xunzhang/dynet,clab/cnn