Add 3rd party web dependency instructions

build/javascript/README.md

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+## Third party web dependencies
+
+Owncast's web frontend utilizes a few third party Javascript and CSS dependencies that we ship with the application.
+
+To add, remove, or update one of these components:
+
+1. Perform your `npm install/uninstall/etc`, or edit the `package.json` file to reflect the change you want to make.
+2. Edit the `snowpack` `install` block of `package.json` to specify what files you want to add to the Owncast project.  This can be an entire library (such as `preact`) or a single file (such as `video.js/dist/video.min.js`).  These paths point to files that live in `node_modules`.
+3. Run `npm run build`.  This will download the requested module from NPM, package up the assets you specified, and then copy them to the Owncast web app in the `webroot/js/web_modules` directory.
+4. Your new web dependency is now available for use in your web code.