Add note about V3 to readme (#626)

README.md

@@ -8,6 +8,10 @@ django-auditlog
 [![Supported Python versions](https://img.shields.io/pypi/pyversions/django-auditlog.svg)](https://pypi.python.org/pypi/django-auditlog)
 [![Supported Django versions](https://img.shields.io/pypi/djversions/django-auditlog.svg)](https://pypi.python.org/pypi/django-auditlog)
 
+**Migrate to V3**
+
+Check the [Upgrading to version 3](https://django-auditlog.readthedocs.io/en/latest/upgrade.html) doc before upgrading to V3.
+
 ```django-auditlog``` (Auditlog) is a reusable app for Django that makes logging object changes a breeze. Auditlog tries to use as much as Python and Django's built in functionality to keep the list of dependencies as short as possible. Also, Auditlog aims to be fast and simple to use.
 
 Auditlog is created out of the need for a simple Django app that logs changes to models along with the user who made the changes (later referred to as actor). Existing solutions seemed to offer a type of version control, which was found excessive and expensive in terms of database storage and performance.