document lightweight dynamic server, link to #585.

README.md

@@ -164,6 +164,7 @@ software works.
 `GA_DEBUG` | When set to 'on' will output verbose info to the console about what data is being sent to Google Analytics.
 `MAILINGLIST_URL` | is the mailinglist signup form URL generated by BSD. If undefined, mailinglist signup is simulated for development purposes.
 `MAILINGLIST_PRIVACY_NAME` | is the `name` attribute of the privacy policy checkbox on the mailinglist signup form generated by BSD.
+`PORT` | is the port to serve the experimental lightweight dynamic server on. Defaults to 8008.
 
 ### Using Environment Variables in Local Development
 
@@ -180,6 +181,27 @@ Then run `source .env` before running `npm start`.
 want to create a batch file that uses
 [`setx.exe`](http://stackoverflow.com/a/11190331).
 
+## Lightweight Dynamic Server (Experimental)
+
+The site can also be served through a lightweight server which
+dynamically generates requested HTML files. This feature is currently
+experimental. For more information on the rationale behind it, see
+[#585][].
+
+To run the server in development mode, use `npm run app`. The
+server-side page rendering code will be dynamically updated as
+relevant files are changed.
+
+For production deployment, you will need to:
+
+1. Set `NODE_ENV=production` and other relevant environment variables
+   and then re-run `npm install`. This will build all static assets
+   that won't change during production.
+
+2. Run `node app.js`.
+
+The server is also designed to be easily deployable on Heroku.
+
 ## References
 
 * [Roadmap][roadmap]
@@ -203,3 +225,4 @@ want to create a batch file that uses
   [changelog]: https://github.com/mozilla/teach.webmaker.org/blob/develop/CHANGELOG.md
   [contributing]: https://github.com/mozilla/teach.webmaker.org/blob/develop/CONTRIBUTING.md
   [Manual Testing]: https://github.com/mozilla/teach.webmaker.org/blob/develop/CONTRIBUTING.md#manual-testing
+  [#585]: https://github.com/mozilla/teach.webmaker.org/issues/585