Add environment-plugin documentation

Source: written by myself

Author: simon04

content/plugins/environment-plugin.md

@@ -0,0 +1,24 @@
+---
+title: environment-plugin
+contributors:
+  - simon04
+---
+
+The environment-plugin is a shorthand for using the [define-plugin](/plugins/define-plugin)
+on [`process.env`](https://nodejs.org/api/process.html#process_process_env) keys.
+
+**Example:**
+
+```javascript
+new webpack.EnvironmentPlugin([
+  'NODE_ENV'
+])
+```
+
+This is equivalent to the following define-plugin application:
+
+```javascript
+new webpack.DefinePlugin({
+  'process.env.NODE_ENV': JSON.stringify(process.env.NODE_ENV)
+})
+```