Add PyPI badge to README
Add PyPI badge to README

Former-commit-id: 2f29050cb8c1045e566f7a21c63c732894f12dd8
clab/dynet,chunyang-wen/dynet,xunzhang/dynet,chunyang-wen/dynet,clab/dynet,shuheik/dynet,shuheik/dynet,xunzhang/dynet,clab/dynet,danielhers/dynet,clab/cnn,danielhers/dynet,clab/cnn,chunyang-wen/dynet,clab/dynet,shuheik/dynet,danielhers/dynet,danielhers/dynet,clab/cnn,shuheik/dynet,danielhers/dynet,clab/dynet,danielhers/dynet,chunyang-wen/dynet,xunzhang/dynet,shuheik/dynet,xunzhang/dynet,danielhers/dynet,shuheik/dynet,clab/dynet,clab/dynet,shuheik/dynet