Add documentation about Play 2 packaging as deb- and rpm-packages

With this PR I'd like to address issue #378. 

Since I'm not a native english speaker I could make some dumb mistakes, so feel free to let me know! And if you feel that something is missing also let me know and I'll try to fix that.
I think this problem was already fixed in #381 . I would change this section to sound a bit more positive. E.g.
> **Upstart** is not supported by all available operation systems and may not always work as expected (See #381). You can always fallback to the **SystemV** service manager instead.