Added readme

Author: samuelmaddock

README.md

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+# CSS Variable Theme Webpack Plugin
+
+## Install
+
+```bash
+npm install --save-dev css-variable-theme-webpack-plugin
+```
+
+## Usage
+
+```js
+/* webpack.config.js */
+const path = require('path');
+const ThemePlugin = require('css-variable-theme-webpack-plugin');
+
+module.exports = {
+  module: {
+    rules: [
+      {
+        test: /\.css$/,
+        use: [
+          'css-loader',
+          path.join(__dirname, './node_modules/css-variable-theme-webpack-plugin/loader')
+        ]
+      }
+    ]
+  },
+  plugins: [
+    new ThemePlugin({
+      themes: {
+        light: 'light.css',
+        dark: 'dark.css'
+      }
+    }),
+  ]
+}
+```
+
+Theme files support CSS variable syntax on the `:root` selector.
+```css
+/* light.css */
+:root {
+    --color: black;
+    --background-color: white;
+}
+
+/* dark.css */
+:root {
+    --color: white;
+    --background-color: black;
+}
+```
+
+Use variables from your theme files in your stylesheets.
+```css
+/* styles.css */
+body {
+    color: theme-var(--color);
+    background-color: theme-var(--background-color);
+}
+```
+
+### Usage with [CSS Modules](https://github.com/css-modules/css-modules)
+
+Use loader options `modules` and `localIdentName` to generate locally scoped CSS class names. These work the same as [css-loader](https://github.com/webpack-contrib/css-loader#css-scope).