Update Changelog to add upgrade node

When upgrading from old versions it's necessary to incrementally upgrade at version `3.0.1`, run migrations, and then proceed.
[See here](#822 (comment))

CHANGELOG.rst

@@ -46,7 +46,16 @@ django-reversion changelog
 ------------------
 
 - Removed squashed migrations, as they subtly messed up the Django migrations framework (@etianen).
-
+
+  To upgrade to ``>= 3.0.2`` from ``< 3.0.1``:
+
+  .. code::
+  
+    pip install django-reversion==3.0.1
+    python manage.py migrate reversion
+    pip install --upgrade django-reversion
+    python manage.py migrate reversion
+ 
 
 3.0.1 - 2018-10-23
 ------------------