update build instructions for dev build

Author: yyx990803

src/guide/installation.md

@@ -37,8 +37,17 @@ NPM is the recommended installation method when building large scale apps with V
 $ npm install vue
 # latest stable + CSP-compliant
 $ npm install vue@csp
-# dev build (directly from GitHub):
-$ npm install vuejs/vue#dev
+```
+
+## Dev Build
+
+**Important**: the CommonJS bundle distributed on NPM (`vue.common.js`) is **not** checked into source control, therefore to use Vue from the latest source code on GitHub, you will have to build it yourself!
+
+``` bash
+git clone https://github.com/vuejs/vue.git node_modules/vue
+cd node_modules/vue
+npm install
+npm run build
 ```
 
 ## Bower