Validation on many to many field when default=None

[Genarito]

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Description

Hi!
First of all, thank you for the library, it's very useful for my work!

While working on a project, I got a 500 error in production due to a configuration error on my part (copy-paste 🤦‍♂️): when using a PrimaryKeySerializer on a ManyToMany field and setting default=None like this:

class ManyToManyTarget(RESTFrameworkModel):
    name = models.CharField(max_length=100)


class ManyToManySource(RESTFrameworkModel):
    name = models.CharField(max_length=100)
    targets = models.ManyToManyField(ManyToManyTarget, related_name='sources')


class ManyToManySourceSerializer(serializers.ModelSerializer):
    targets = serializers.PrimaryKeyRelatedField(
        many=True,
        queryset=ManyToManyTarget.objects.all(),
        default=None  # <---- This
    )

It crashes at runtime with the following exception when no value is provided for targets field:

my_file.py:XXX: in some_func
    instance = serializer.save()
rest_framework\serializers.py:210: in save
    self.instance = self.create(validated_data)
rest_framework\serializers.py:1017: in create
    field.set(value)
venv\lib\site-packages\django\db\models\fields\related_descriptors.py:1328: in set
    objs = tuple(objs)

Now it seems simple and intuitive, but in production, with thousands of lines, dozens of serializers, and hundreds of fields, it took me a long time to figure out what the problem was.
To prevent errors in production, I added a new validation in the get_fields() method with a super descriptive message indicating a solution (I also added some tests to check this new behavior).
I couldn't find an earlier place to perform the validation, but this change allows the validation to run for other methods such as create() or update(). If you have a better place where the code block could be moved, please let me know, and I'll make the necessary adjustments.

Finally, since this is my first time collaborating on the project, I added some help to the contributing.md file to explain a few things, such as the -s parameter in pytests, which allows you to obtain the output during testing, or django.test.TestCase inheritance so you can use the Django DB. If you think it shouldn't be there, or you don't like the format, let me know and I can make the changes.

Thanks and best regards!