Adding a CONTRIBUTING.md about django-sass-processor

[jrief]

Hi @AndreasBackx ,
this may seem weird, but I'm using the quoting style formerly proposed by Douglas Crockford:
"Single quotes for internal strings and double quotes for strings readable by humans".
This can be a big advantage, whenever one has to search for strings visible in other natural languages.
You therefore might see statements in my code such as:

color = ChoiceField(choices=[('red', "Red"), ('green', "Green"), ('blue', "Blue")])

This of course also applies to docstrings:

def foo():
    """Do foo stuff"""

About the line length, I follow the guideline from Django, which allows 119 characters, but I use a soft limit of 100, which means that I try to stay below, if otherwise I would have to use a \ character.

I fully agree, that this should be added to a contributing file.

[AndreasBackx]

Good to know that you use that quoting style. I don't really mind and will do what has been done in the past on the project. Mine were only suggestions in case there weren't any.

I would also suggest using isort for imports.

[AndreasBackx]

No CONTRIBUTING.md I guess?