add STXXL citation

Author: mdimura

README.md

@@ -1,4 +1,4 @@
-Sparsehc-dm is a python wrapper for sparcehc distance matrix clustering algorithm, integrated with STXXL for on-disk sorting. SparseHC [![DOI for Citing SparseHC](https://img.shields.io/badge/DOI-10.1016%2Fj.procs.2014.05.001-blue.svg)](http://dx.doi.org/10.1016/j.procs.2014.05.001) is a memory-efficient hierarchical agglomerative clustering implementation. It has close to linear memory complexity, enabling clustering of ~900000 structures/points on 32GB RAM.
+Sparsehc-dm is a python wrapper for sparcehc distance matrix clustering algorithm, integrated with STXXL[![DOI for Citing STXXL](https://img.shields.io/badge/DOI-10.1002%2Fspe.v38%3A6-blue.svg)](http://dx.doi.org/10.1002/spe.v38:6) for on-disk sorting. SparseHC [![DOI for Citing SparseHC](https://img.shields.io/badge/DOI-10.1016%2Fj.procs.2014.05.001-blue.svg)](http://dx.doi.org/10.1016/j.procs.2014.05.001) is a memory-efficient hierarchical agglomerative clustering implementation. It has close to linear memory complexity, enabling clustering of ~900000 structures/points on 32GB RAM.
 
 Usage example:
 ```python

examples/fastcluster_load.py

@@ -2,7 +2,7 @@
 import sys
 import time
 import scipy.cluster as cluster
-from scipy.spatial.distance import squareform
+#from scipy.spatial.distance import squareform
 from fastcluster import linkage
 import mdtraj as md
 import numpy as np