README: document webpack asset management

{{cookiecutter.app_name}}/README.rst

@@ -40,6 +40,23 @@ database tables and perform the initial migration ::
     flask db upgrade
     flask run
 
+Files placed inside the ``assets`` directory and its subdirectories
+will be copied by webpack's ``file-loader`` into the ``static/build``
+directory, with hashes of their contents appended to their names.  For
+instance, if you have the file ``assets/img/favicon.ico``, this will
+get copied into something like
+``static/build/img/favicon.fec40b1d14528bf9179da3b6b78079ad.ico``.
+You can then put this line into your header::
+
+    <link rel="shortcut icon" href="{{asset_url_for('img/favicon.ico') }}">
+
+to refer to it inside your HTML page.  If all of your assets are
+managed this way, you can ask Flask to tell web browsers that they
+should cache your static files forever by including the following line
+in your ``settings.py``::
+
+    SEND_FILE_MAX_AGE_DEFAULT = 31556926  # one year
+
 
 Deployment
 ----------