point readme badges to dockerhub in r-notebook

* status, pulls badges now point to https://hub.docker.com/r/jupyter/r-notebook/
* microbadger badges now have 'image metadata' alt text

r-notebook/README.md

-![docker pulls](https://img.shields.io/docker/pulls/jupyter/r-notebook.svg) ![docker stars](https://img.shields.io/docker/stars/jupyter/r-notebook.svg) [![](https://images.microbadger.com/badges/image/jupyter/r-notebook.svg)](https://microbadger.com/images/jupyter/r-notebook "jupyter/r-notebook image metadata")
+[![docker pulls](https://img.shields.io/docker/pulls/jupyter/r-notebook.svg)](https://hub.docker.com/r/jupyter/r-notebook/) [![docker stars](https://img.shields.io/docker/stars/jupyter/r-notebook.svg)](https://hub.docker.com/r/jupyter/r-notebook/) [![image metadata](https://images.microbadger.com/badges/image/jupyter/r-notebook.svg)](https://microbadger.com/images/jupyter/r-notebook "jupyter/r-notebook image metadata")
 
 # Jupyter Notebook R Stack