Issue #811. Document what building does for what and when.

CONTRIBUTING.md

@@ -20,7 +20,7 @@ You are welcome to contribute to this project. Here are the guidelines we try to
     * [Installing Grunt](#installing-grunt)
   * [Configuring The Server](#configuring-the-server)
   * [Starting The Server](#starting-the-server)
-  * [Building Project](#building-project)
+  * [Building the Project](#building-the-project)
 * [Coding](#coding)
 * [Running Tests](#running-tests)
   * [Functional Tests](#functional-tests)
@@ -333,7 +333,14 @@ make start
 
 You should now have a local instance of the site running at `http://localhost:5000/`. Please [file bugs](https://github.com/webcompat/webcompat.com/issues/new) if something went wrong!
 
-### Building Project
+### Building the Project
+
+You need to build the project before serving it from a webserver will work, and after certain kinds of changes are made.
+
+* CSS: a build will run cssnext, combine custom media queries, and concat all source files into webcompat.dev.css. You'll need to re-build the CSS to see any changes, so it's recommended to use a watch task (see `make watch` or `grunt watch`).
+* JS: a build will run eslint, minify and concat source files.
+* HTML templates: the changes should be served from disk without the need for rebuilding
+* Python: the Flask local server will detect changes and restart automatically. No need to re-build.
 
 You can build the entire project (CSS and JavaScript files and optimize images) by executing this command on Mac/Linux: