Adding examples and documentation for caching

[pratyushmittal]

The default caching system of Django works very well with DRF. However, the usage is not very easy to understand for class based views as the documentation is spread across pages. This pull-request provides example code on how to use caching with apiviews and viewsets.

(Edit: @carltongibson: Closes #370)

[carltongibson]

Thanks for this. I will give it a proper review tomorrow AM.

(I think there may be some long standing issues related to this... I need to have a look)

[carltongibson]

@pratyushmittal It turns out the issue was #370 — our very oldest open issue! So you get a big cheer for that: 🎉💃🏼🎇

This is a good beginning to a caching page. No doubt people will follow-up with more but this is what we need to get started.

Awesome! Thank you!

[pratyushmittal]

Thanks a lot @carltongibson for a super quick review, merge and appreciation :-)