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

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

Backport of r12560 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@12565 bcc190cf-cafb-0310-a4f2-bffc1f526a37