Point badges to dockerhub, add alt-text to microbadger

Updates:

* Currently clicking on the pulls & stars badges directs users to the actual button svg image source. The link should point to https://hub.docker.com/r/jupyter/minimal-notebook/.
* Add alt-text "image metadata" to microbadger badge

minimal-notebook/README.md

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