Document dependencies in README.

Document (hopefully) all non-standard dependencies:

- Python 2.7
- SQL Database
- OpenLDAP client libraries
- Java
- LaTeX
- HAProxy
- Apache
- Postfix
- Perl and Email::Outlook::Message module
- Celery, Erlang and RabbitMQ

@phgross @deiferni 
(See [rich diff](https://github.com/4teamwork/opengever.core/pull/635/files?short_path=88b99bb#diff-88b99bb28683bd5b7e3a204826ead112) for easier reviewing)