In the contributing guide document how to deprecate a feature

[Tim Graham]

Somewhere in internals/contributing we document guidelines for deprecating a feature. I couldn't find anything on this after a quick search.

This includes how to silence deprecation warnings:

When a feature is deprecated, it raises a PendingDeprecationWarning, either at import time or at run time. After the following release it raises DeprecationWarning. Assuming we have a good test coverage, these warnings will be shown by the test suite, with -Wall for PendingDeprecationWarning, or by default for DeprecationWarning. This is annoying. The output of the test suite should remain clean.

Aymeric's full post including how to silence warnings:
​https://groups.google.com/forum/#!topic/django-developers/nXlNqpy6sC8

[Aymeric Augustin]

#21025 was a duplicate.

[Marc Tamlyn]

Worth mentioning that we should make sure to take care when silencing deprecation warnings - eating our own dogfood must be the first step to removing the warnings.

[Keith Edmiston]

I think I may have found the location in the docs to be in the Minor Releases section of the ​Release Process page.

[Ivan Kolodyazhny]

"The first step is to remove any use of the deprecated behavior by Django itself"
What's about cases when some code doesn't used by Django itself? Should we write any tests in this case?

[Tim Graham]

I'm not sure how common that would be. I guess I would handle that on a case by case basis.

[Tim Graham <timograham@…>]

In 5be56d0e0d8492a41b3d40757a4f2677211a9179:

Fixed #21024 -- Documented how to deprecate a feature.