Asset builder example

[aydun]

Add step-by-step example
@totten - thanks for the train station tutorial!

[totten]

I like how "CSS Example" shows the declaration (hook_civicrm_buildAsset) and then shows the command to preview it (cv ev ...). We should probably update "Usage: Simple asset" to do something similar.

On second thought, a more natural sequencing might be:

1. Declare the asset (hook_civicrm_buildAsset)
2. Preview the asset (cv ev ... )
3. Use the asset at runtime