Add documentation.

src/index.js

@@ -7,6 +7,8 @@
  * When the bundle created from this file is imported in the server
  * side, the exported `renderApp` function can be used for server side
  * rendering.
+ *
+ * Note that this file is required for the build process.
  */
 
 import React from 'react';

src/styles/variables.js

@@ -1,3 +1,37 @@
+/**
+ * This file exports variables that can be shared for both CSS and JS.
+ *
+ * The build process sets this the exports from this file as the
+ * variables for postcss-custom-properties. These variables should
+ * **not** have the -- prefix and cannot be overridden in CSS files.
+ *
+ * See: https://github.com/postcss/postcss-custom-properties
+ *
+ * Example when this file would export a `baseColor` value:
+ *
+ * In JavaScript, the module should be imported:
+ *
+ *     import cssVariables from '../../styles/variables';
+ *
+ *     const MyReactComponent = (props) =>
+ *       <p style={{ color: cssVariables.baseColor }}>hello, world</p>;
+ *
+ * In CSS, no import is required:
+ *
+ *     .title {
+ *       color: var(--baseColor);
+ *     }
+ *
+ * This file should only be used for variables that are needed in both
+ * JS and CSS. As this file is only imported in the start of the build
+ * process, changes to this file require restarting the (dev)
+ * server. For variables only needed in CSS, a separate CSS file with
+ * those variables can be used to support hot loading changes with no
+ * need to restart.
+ *
+ * Note that this file is required for the build process.
+ */
+
 module.exports = {
 
 };