update README section about serialization
update README section about serialization


Former-commit-id: 93842c939f2af9f51ffb5b14dc58975e90e79b91
chunyang-wen/dynet,shuheik/dynet,xunzhang/dynet,danielhers/dynet,danielhers/dynet,danielhers/dynet,chunyang-wen/dynet,chunyang-wen/dynet,clab/cnn,shuheik/dynet,clab/dynet,clab/dynet,clab/dynet,clab/dynet,shuheik/dynet,danielhers/dynet,xunzhang/dynet,shuheik/dynet,clab/cnn,clab/dynet,xunzhang/dynet,shuheik/dynet,danielhers/dynet,shuheik/dynet,clab/cnn,chunyang-wen/dynet,danielhers/dynet,clab/dynet,shuheik/dynet,xunzhang/dynet,clab/dynet,danielhers/dynet