idea_plugin README: Add macOS menu location to instructions
idea_plugin README: Add macOS menu location to instructions

Fixes #199

MOE_MIGRATED_REVID=170553006

sormuras/google-java-format,cushon/google-java-format,cushon/google-java-format,sormuras/google-java-format,sormuras/google-java-format,cushon/google-java-format