Add LICENSE and appropriate credit

[keimlink]

Currently the tutorial does not contain a LICENSE. This means all content can't be changed, used and published by others without explicit allowance by the original authors. This is even true for all people that contributed so far.

Because I believe that this is not the intention of the original authors a LICENSE file should be added to the repository.

Parts of the tutorial use content from other tutorials. One of them is my Django tutorial django-marcador. It is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License. In my opinion there are two things that need to be changed to meet the requirements of the license:

1. The way credit is given does not match the terms of the CC-BY-SA 4.0 license. it says that a user must "provide the name of the creator and attribution parties, a copyright notice, a license notice, a disclaimer notice, and a link to the material".
2. Because the license is a "ShareAlike" license the Django Girls tutorial must also be licensed under a CC-BY-SA 4.0 license.

License and proper credits could look like this:

# License

This work is licensed under the Creative Commons Attribution-ShareAlike 4.0
International License. To view a copy of this license, visit
http://creativecommons.org/licenses/by-sa/4.0/ or send a letter to Creative
Commons, 444 Castro Street, Suite 900, Mountain View, California, 94041, USA.

## Authors

- Ola Sitarska
- Ola Sendecka
- Baptiste Mispelon
- ...

## Credits

Parts of this work are based on the [django-marcador
tutorial](http://django-marcador.keimlink.de/) licensed under Creative Commons
Attribution-ShareAlike 4.0 International License. The django-marcador tutorial
is copyrighted by Markus Zapke-Gründemann et al.

Of course you can split the authors section into main authors and contributors if you want to.

[keimlink]

Thanks @asendecka!