Readme update on reversions

Hmmm, we're realising that there are some problem with reversions even though it's definitely the most widely used. See:

https://groups.google.com/forum/#!topic/django-reversion/kitOQtlXfgs

<= A pain in the arse and we think this is still extant.

Maybe we should look at this again:

https://www.djangopackages.com/grids/g/model-audit/