update README with cluster scoring

Author: lmcinnes

README.rst

@@ -78,7 +78,9 @@ to Pandas or NetworkX for further analysis. See the notebook on
 
 The clusterer objects also have an attribute providing cluster membership
 strengths, resulting in optional soft clustering (and no further compute 
-expense)
+expense). Finally each cluster also receives a persistence score giving
+the stability of the cluster over the range of distance scales present
+in the data. This provides a measure of the relative strength of clusters.
 
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 Outlier Detection