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.

Comece pela seção Eclipse de README.md e reproduza o comportamento relatado usando as configurações de Active profile do Eclipse, com a implementação google-java-format selecionada. Documente o que a configuração do formatter e Active profile controlam, incluindo se as alterações de perfil afetam Source / Format, para que README indique claramente qual perfil escolher.

Escrita pelo modelo de indexação a partir do texto da issue.