Proposed fix for #18162.
One last final fix for 0ad76843. Tested on py3 and py2
Fixed #19698 -- Cleared sites cache with signals

Thanks ddavies at nomensa.com for the report and the patch and
serdaroncode for reviewing the patch.
Added a draft document explaining how to release Django.

Thanks to James for the first draft; I made a few changes (svn->git) and
some supporting links, but mostly I added FIXME's.
Fixes #17866: Vary: Accept-Language header when language prefix used
Some updates to "how to release Django":

Typo fixes, spell check, some more specifics where possible.
Fixed #19775 - Clarified requirements of the "default" database.

Thanks monkut for the report and wsmith323 for the patch.
Fixed #19880 - Fixed an error in the form wizard initial_dict example.

Thanks almalki for the report.
Merge pull request #796 from ambv/vary

Fixes #17866: Vary: Accept-Language header when language prefix used
Fixed #19579 - Documented that "providing_args" is purely documentational.
Fixed 19895 -- Made second iteration over invalid queryset raise an exception too

When iteration over a queryset raised an exception, the result cache
remained initialized with an empty list, so subsequent iterations returned
an empty list instead of raising an exception
Fixed #19402 - Clarified purpose of CustomUser.REQUIRED_FIELDS

Thanks pydanny for the report and ptone for the patch.
Merge pull request #804 from oinopion/ticket18162

Fixes #18162 --  Specifying choices to Field overrides any form_class argument
Fixed #19282 -- Restored ability to pass Decimals to intcomma filter
Added an example of "default" database dictionary left blank; refs #19775.

Thanks wsmith323 for the patch.
Some style fixes for stuff that was clearly brough tover from doctests.
Fixed #18829 -- Fixed ModelChoiceIterator length

Thanks facundo.olano at gmail.com for the report and thikonom for
the initial patch.