Document how to minimize JS when using aio app:deploy

[sarahxxu]

Forking @moritzraho 's answer here to be documented --

Developer can replace our build script with a build-static hook in the package.json scripts field. https://www.adobe.io/apis/experienceplatform/project-firefly/docs.html#!AdobeDocs/project-firefly/master/guides/app-hooks.md

We did not minify by default because this was introducing bugs in some cases. However, we do tree shaking (remove uneeded code in deps) and this has shown to reduce the size already enough in most cases.

[aiojbot]

JIRA issue created: https://jira.corp.adobe.com/browse/AIOE-475