Add warning of numpy performance regression to README.

Author: lmcinnes

README.rst

@@ -16,6 +16,12 @@ HDBSCAN is ideal for exploratory data analysis; it's a fast and robust
 algorithm that you can trust to return meaningful clusters (if there
 are any).
 
+**WARNING**: Numpy version 1.10.1 introduced `performance regressions in structured
+array access <https://github.com/numpy/numpy/issues/6467>`_ ; this does have
+a notable performance impact on hdbscan. Consider upgrading to a newer version,
+or downgrading in the meantime if performance counts. I will be working to
+ensure hdbscan performance regardless of this issue in coming releases.
+
 Based on the paper:
     R. Campello, D. Moulavi, and J. Sander, *Density-Based Clustering Based on
     Hierarchical Density Estimates*