improvements to CONTRIBUTING.md

Author: lucacasonato

CONTRIBUTING.md

@@ -4,6 +4,9 @@ To build the spec locally first install bikeshed:
 pip3 install bikeshed && bikeshed update
 ```
 
+Then to build the spec (index.bs) into HTML (index.html), run one of the below
+commands:
+
 ```sh
 bikeshed spec # build once