Update Usage docs

[Querschlag]

I played with OCMapView a while ago and came back to use it in my new project. The documentation set's you up really simple with comment like

"Just replace your MKMapView references with OCMapView. It automatically handles annotations and combines them to clusters for you without any other change of your code."

Though I struggled to begin with the actual clustering and it hits me again this time. It is important to add

- (void)mapView:(MKMapView *)aMapView regionDidChangeAnimated:(BOOL)animated {
    [self.mapView doClustering];
}

which is not mentioned in the docs. You might consider adding this to get new users a better start with OCMapView.

[BotiKis]

You are right, i should mention it in the notes! Thanks for the feedback.