📝 Mention build-essential package

Closes #2384

docs/build-instructions/linux.md

@@ -5,6 +5,8 @@ Ubuntu LTS 12.04 64-bit is the recommended platform.
 ## Requirements
 
   * OS with 64-bit or 32-bit architecture
+  * C++ build toolchain
+    * on Ubuntu/Debian:  `sudo apt-get install build-essential`
   * [node.js](http://nodejs.org/download/) v0.10.x
   * [npm](http://www.npmjs.org/) v1.4.x  
   * libgnome-keyring-dev
@@ -15,7 +17,6 @@ Ubuntu LTS 12.04 64-bit is the recommended platform.
     * This command may require `sudo` depending on how you have
       [configured npm](https://github.com/joyent/node/wiki/Installing-Node.js-via-package-manager#ubuntu-mint-elementary-os).
 
-
 ## Instructions
 
   ```sh