Document Indentation Style Differences

The README mentions the four indentation styles specific to PHP Mode but provides no detailed description of the differences. We basically assume the user is already familiar with the styles of projects like WordPress and Drupal. It will be better if we link to thorough descriptions of those styles so that users can know what to expect if they choose, e.g., to use the `symphony2` style.

Having references to style guides will also be useful when it comes to fixing incorrect behavior for any style.