chore(docs): update readme with --appPath option usage example

This trivial PR intends to complete documentation section for application output configuration with added example use of `--appPath` option. 
When used in non-intended way `--appPath` can lead to cryptic errors that do not explain reason of generator error.
For details please see: https://github.com/yeoman/generator-angular/issues/926
Generator v. 0.10.0
![screen shot 2014-11-15 at 00 39 09](https://cloud.githubusercontent.com/assets/14539/5055215/6aedebe0-6c60-11e4-9419-4bd792daa770.png)