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

[amiroff]

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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.

Steps to reproduce

1. Read the docs.

Actual behavior

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

Expected behavior

Fixed with #314