docs: how to use with i18n_patterns

[romain-dartigues]

Using django-hosts together with the Djangos language prefix in URL patterns feature is a common use-case; unfortunately it is not straight-forward as using i18n_patterns within an included URLconf will throw an ImproperlyConfigured exception.

Here I suggest an implementation example, but I'm probably missing some points and I would like your enlightenment.

I also would like to add unit tests concerning this topic, but I was unable to setup the testing environment properly; I also kindly request your guidance on it.

Relates to:

• Using django-hosts on multi-language site #68
• https://stackoverflow.com/q/66939490/248390
• https://stackoverflow.com/q/56389501/248390