solve react-toolbox#1444 and react-toolbox#1359. (react-toolbox#1587)

* update webpack config to make "Basic usage" work.

I use webpack version 2.6 and new configure syntax rather than 1.x.

When I follow the instruction on `readme.md`. It always failed on *Basic Usage* with the message below.
``` bash
ERROR in ./~/css-loader?{"modules":false,"importLoaders":1,"localIdentName":"[name]--[local]--[hash:base64:8]"}!./~/postcss-loader/lib!./~/react-toolbox/lib/button/theme.css
  Module build failed: Error: composition is only allowed when selector is single :local class name not in ".raised", ".raised" is weird
```

So I figured out what's wrong and update `reame.md` to notify others.

* edit multiple number.

According to docs, the width is larger than 320px and if the device is mobile than 280px. To follow this specification It should be 5, not 4.

README.md

@@ -58,6 +58,26 @@ Declare plugins to be used by postcss (as part of webpack's config object):
   },
 ```
 
+Configure webpack 2.x or 3.x loader for .css files to use postcss:
+```js
+      {
+        test: /\.css$/,
+        use: [
+          "style-loader",
+          {
+            loader: "css-loader",
+            options: {
+              modules: true, // default is false
+              sourceMap: true,
+              importLoaders: 1,
+              localIdentName: "[name]--[local]--[hash:base64:8]"
+            }
+          },
+          "postcss-loader"
+        ]
+      }
+```
+
 ## Basic usage
 
 In this minimal example, we import a `Button` with styles already bundled:

components/drawer/config.css

@@ -3,6 +3,6 @@
   --drawer-border-color: var(--palette-grey-300);
   --drawer-text-color: var(--palette-grey-800);
   --drawer-width: calc(24 * var(--unit));
-  --drawer-desktop-width: calc(4 * var(--standard-increment-desktop));
+  --drawer-desktop-width: calc(5 * var(--standard-increment-desktop));
   --drawer-mobile-width: calc(5 * var(--standard-increment-mobile));
 }