Document how to get redirect from

Author: calpaterson

README.md

@@ -11,7 +11,7 @@ https://helsinki-python.github.io
 ## Building
 
 ```bash
-sudo apt install jekyll ruby-jekyll-sitemap
+sudo apt install jekyll ruby-jekyll-sitemap ruby-jekyll-redirect-from
 make # this will serve on http://localhost:4000
 ```