Fixed #12538 -- Added a note that pickles aren't stable during version...

Fixed #12538 -- Added a note that pickles aren't stable during version updates. Thanks to snow0x2d0 for the suggestion.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12560 bcc190cf-cafb-0310-a4f2-bffc1f526a37