Please document the role of Active profile in Eclipse configuration

The [README](https://github.com/google/google-java-format/blob/master/README.md#eclipse) states: “The plugin adds a google-java-format formatter implementation that can be configured in Window > Preferences > Java > Code Style > Formatter > Formatter Implementation.”

It is unclear what can be configured exactly, or whether anything can be configured, considering the note in the same file: “There is no configurability as to the formatter's algorithm for formatting. This is a deliberate design decision to unify our code formatting on a single format.”

Relatedly, the role of the Active profile dialog box is not clear.

I just tested changing the number of spaces in the active profile, and this seems to have no effect on `Source / Format` when the google-java-format implementation is selected.

Please add some explanations about what can be configured and which Active profile should be chosen or indicate if it is supposed to have no effect.