Add npm install to usage commands.

Author: binki

README.md

@@ -7,5 +7,6 @@ This is an example of a basic Javascript implementation of a bot for [generals.i
 ```
 git clone https://github.com/vzhou842/generals.io-Node.js-Bot-example.git
 cd generals.io-Node.js-Bot-example
+npm install
 node main.js
 ```