add syntax highlight in the python examples

[Maksim-Burtsev]

As we know Markdown support syntax highlight in many languages and this makes docs more readable and pretty:

Before

from mptt.models import MPTTModel, TreeForeignKey

class Category(MPTTModel):
    ...
    parent = TreeForeignKey('self', null=True, blank=True)
    slug = models.SlugField()
    class Meta:
        unique_together = ('slug', 'parent')

After

from mptt.models import MPTTModel, TreeForeignKey

class Category(MPTTModel):
    ...
    parent = TreeForeignKey('self', null=True, blank=True)
    slug = models.SlugField()
    class Meta:
        unique_together = ('slug', 'parent')

So i update this in readme =)

[0xE111]

This is so nice, thank you very much!