Adding documentation about beans.xml. Closes #815.

you can create any reusable selector (class) and a simple css style for it.

For instance, on the page:
```
Some text here
{: .custom-box}
```
and at `style.css` you can do:

```
.custom-box {
   # some beauty css to the box 
}
```
If you prefer you can keep it as it is... if needed we can customize in the future