Contribution Guideline

- [x] Branch Policy
- [x] [Coding Standard](https://github.com/guitar-academy/gaclient/issues/2)
- [x] Testing
- [x] Developer Documentation
- [x] Contribution Guide

Anything else?
I just notice the Google convention for css was actually 2. I am used to 4 spaces. 
My rationale is because CSS isn't like HTML nesting. HTML is nested to the right direction while CSS is usually flatten by classes rather than nested. I hardly write CSS more than 79 char per line. So I put more indentation to make it clear distinction.

```
<div class='container'>
   <div class='sidebar'>
       <div class='menu'>
</div></div></div>
```
while css
```
.menu {
   color: #fff;
   weight: heavy;
}
.container {
   color: #aaa;
}

.sidebar {
   weight: light;
}

```