docs: Add PostCSS instructions

Resolves #38

docs/template-engine/styling.md

@@ -84,7 +84,7 @@ For external CSS content, use `style` helper method from the result of the `html
   * **returns**:
     * update function compatible with content expression 
 
-Style helper works the best with bundlers, which support importing text content of the CSS files (for [Webpack](https://github.com/webpack/webpack) use [css-loader](https://github.com/webpack-contrib/css-loader) without [style-loader](https://github.com/webpack-contrib/style-loader)):
+Style helper works the best with bundlers, which support importing text content of the CSS files (for [webpack](https://github.com/webpack/webpack), use [raw-loader](https://github.com/webpack-contrib/raw-loader).
 
 ```javascript
 // `styles` should contain text content of CSS file
@@ -96,3 +96,56 @@ const MyElement = {
   `.style(styles),
 };
 ```
+
+### webpack Config
+
+```js
+module.exports = {
+  module: {
+    rules: [
+      {
+        test: /\.css$/i,
+        use: ['raw-loader']
+      }
+    ]
+  }
+}
+```
+
+## Preprocessors (PostCSS, Sass, etc.)
+
+If using external stylesheets (above), you can add a preprocessor such as [PostCSS](https://github.com/postcss/postcss) using a webpack config. You can alternately use `"sass-loader"` for Sass.
+
+It’s important **not** to use `css-loader` or `style-loader` like you may be used to, as either will interfere with Hybrid’s ability to parse the stylesheet.
+
+### Installation
+
+```bash
+npm install --save-dev raw-loader postcss postcss-loader postcss-preset-env
+```
+
+### webpack Config
+
+```js
+const postcssPresetEnv = require("postcss-preset-env"); // optional; for example (below)
+
+module.exports = {
+  module: {
+    rules: [
+      {
+        test: /\.css$/i,
+        use: [
+          'raw-loader',
+          {
+            loader: 'postcss-loader',
+            // This is optional; just showing an example of a plugin w/ options
+            options: {
+              plugins: () => [postcssPresetEnv({ 'nesting-rules': true })]
+            }
+          }
+        ]
+      }
+    ]
+  }
+}
+```