Add define-plugin documentation

Copy of https://github.com/webpack/docs/wiki/list-of-plugins

Author: simon04

content/plugins/define-plugin.md

@@ -0,0 +1,58 @@
+---
+title: DefinePlugin
+---
+
+``` javascript
+new webpack.DefinePlugin(definitions)
+```
+
+The `DefinePlugin` allows you to create global constants which can be configured at **compile** time. This can be very useful for allowing different behaviour between development builds and release builds.  For example, you might use a global constant to determine whether logging takes place; perhaps you perform logging in your development build but not in the release build.  That's the sort of scenario the `DefinePlugin` facilitates.
+
+Example:
+
+``` javascript
+new webpack.DefinePlugin({
+	PRODUCTION: JSON.stringify(true),
+	VERSION: JSON.stringify("5fa3b9"),
+	BROWSER_SUPPORTS_HTML5: true,
+	TWO: "1+1",
+	"typeof window": JSON.stringify("object")
+})
+```
+
+``` javascript
+console.log("Running App version " + VERSION);
+if(!BROWSER_SUPPORTS_HTML5) require("html5shiv");
+```
+
+Each key passed into `DefinePlugin` is an identifier or multiple identifiers joined with `.`.
+
+* If the value is a string it will be used as a code fragment.
+* If the value isn't a string, it will be stringified (including functions).
+* If the value is an object all keys are defined the same way.
+* If you prefix `typeof` to the key, it's only defined for typeof calls.
+
+The values will be inlined into the code which allows a minification pass to remove the redundant conditional.
+
+Example:
+
+``` javascript
+if (!PRODUCTION)
+	console.log('Debug info')
+if (PRODUCTION)
+	console.log('Production log')
+`````
+After passing through webpack with no minification results in:
+
+``` javascript
+if (!true)
+	console.log('Debug info')
+if (true)
+	console.log('Production log')
+```
+
+and then after a minification pass results in:
+
+``` javascript
+console.log('Production log')
+```