Improve README for getting started

As [suggested](https://github.com/triAGENS/guacamole/issues/64) by @tisba we should hint to the usage of Guacamole in an existing Rails app with another ORM already installed.

The issue with the verbose logging, cannot be solved by Guacamole but needs to [be addressed by Ashikawa::Core](https://github.com/triAGENS/ashikawa-core/pull/124).