Print contributing guidelines when installing in 'develop' mode

# Problem
So Github has a useful feature where it shows the contributing guidelines when you open an issue or pull request: https://github.com/blog/1184-contributing-guidelines.  I can see them right now:

![image](https://f.cloud.github.com/assets/80975/205064/c0eb18e4-8196-11e2-829d-a74450939471.png)

In my view, this is too late though.  By the time you are submitting the pull request, you have already done the work and may have missed crucial information.

# Solution?
I have modified `setup.py` to print some contributing instructions when Oscar is installed in `develop` mode.  You see something like this:

```bash
$ ./setup.py develop
running develop
running egg_info
writing requirements to django_oscar.egg-info/requires.txt
writing django_oscar.egg-info/PKG-INFO
writing top-level names to django_oscar.egg-info/top_level.txt
writing dependency_links to django_oscar.egg-info/dependency_links.txt
reading manifest file 'django_oscar.egg-info/SOURCES.txt'
reading manifest template 'MANIFEST.in'
writing manifest file 'django_oscar.egg-info/SOURCES.txt'
running build_ext
Creating /Users/dwinterbottom/.virtualenvs/oscar/lib/python2.7/site-packages/django-oscar.egg-link (link to .)
django-oscar 0.5-pre-alpha is already the active version in easy-install.pth

...

Finished processing dependencies for django-oscar==0.5-pre-alpha

==================================================================================
You're installing Oscar in 'develop' mode so I presume you're thinking
of contributing:

(a) That's brilliant - thank you for your time
(b) If you have any questions, please use the mailing list:
    django-oscar@googlegroups.com
(c) There are more detailed contributing guidelines that you should have a look at:
    http://django-oscar.readthedocs.org/en/latest/contributing.html

Happy hacking!
==================================================================================
```

Is this a good thing?