Avoid or document the need to run `make build` when modifying static assets

Currently `make build` needs to be run locally when making changes to static assets and the minified assets are placed in the source tree rather than a separate `build` directory. This is not immediately obvious or documented in the README. It is also surprising coming from our other apps where minified assets are kept separate from the source tree.

My preference here would be to not minify assets at all for local development and only minify as part of the Docker image build, as we do for our other apps. A more minimal change is to make sure the steps to update static assets are documented both in the README and the description output when running `make` with no arguments.