Docs do not contain any details on usage in templates (PR available)

Checklist

[x] This issue is not about installing previous versions of django-smart-selects older than 1.2.8. I understand that previous versions are insecure and will not receive any support whatsoever.
[x] I have verified that that issue exists against the master branch of django-smart-selects.
[x] I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
[x] I have debugged the issue to the smart_selects app.
[x] I have reduced the issue to the simplest possible case.
[x] I have included all relevant sections of models.py, forms.py, and views.py with problems.
[x] I have used GitHub Flavored Markdown to style all of my posted code.

Docs do not contain information on how to display related selects in templates.

Fixed with #314