Document yarn and webpack

README.md

@@ -11,6 +11,14 @@ for development
     $ pip install -r requirements.txt
     $ pip install -r requirements-dev.txt
 
+To compile the CSS stylesheets and JS scripts, install npm, then run
+the following commands to run yarn (to fetch and install dependencies) and
+webpack (to compile local assets)
+
+    $ npm install -g yarn webpack
+    $ yarn install
+    $ webpack
+
 ## Settings
 
 Create a file in `project/settings/env.py` which contains django