Update style guide with HAML comment

I don't think I did a great job of explaining the following in the README, so here is a more complete description:

Prefer:
```
%p
  = post.body
```
to
`%p= post.body`

Why:
1. The code appears easier to read. 
2. The HTML is more consistent and easier to read. The HAML compiles into:

```
<p>
  This is some content of a post
</p>
```

`<p>This is some content of a post</p>`