missing "mkdocs-material" added

The Docker instructions were updated, but not the README

Author: mattray

README.md

@@ -5,13 +5,13 @@ This uses [`mkdocs`](https://www.mkdocs.org/)
 Install it with:
 
 ```
-pip install mkdocs
+pip install mkdocs-material
 ```
 
 If you have Python3, you may need to run
 
 ```
-pip3 install mkdocs
+pip3 install mkdocs-material
 ```
 
 ## Serve the site locally