Updated Getting Started instructions to reflect new brunch syntax

Updated the Getting Started section of the README because brunch no longer requires the `--skeleton` flag.

```
% brunch new Testapp --skeleton https://github.com/wordofchristian/brunch-with-hampsters
`--skeleton` option has been removed from `brunch new`.

The syntax is now simply:

brunch new <path-or-URI> [optional-output-dir]
brunch new github:brunch/dead-simple
brunch new gh:paulmillr/brunch-with-chaplin
```