Add notice to README on repository status

[mizvyt]

I suggest adding a notice to README with information on the current maintenance status of the repository, as it seems to not have received updates for quite a while now; links to alternatives or forks would be good to have.

@axiak

[ag-michael]

For others, so you don't waste your time like myself, try this fork: https://github.com/prashnts/pybloomfiltermmap3