Docs, application customization

[saschalalala]

I am currenty using django-survey as an app inside another project. I wanted to modify the templates and classes a little in order to, e.g. insert a captcha into the survey etc. These things weren't that hard to achieve, but I'd consider it a more advanced topic because one needs to know about template inheritance order and settings such as TEMPLATES and DIRS (inside TEMPLATES) for template related things and class inheritance and URL urlpatterns for form and view extensions.

I basically followed this approach: https://medium.com/@_RStein_/extending-third-party-django-app-without-touching-its-code-d35fb136c08f to extend some of the functionality of this app and as explained above, I think it would be a good idea to document this in the Readme.