add some documentation on the histogram representation

Author: BrandonArp

Histograms.md

@@ -0,0 +1,24 @@
+Histograms
+==========
+
+MAD primarily computes histograms.  This document details the implementation of the histograms created
+by MAD.
+
+Representation
+--------------
+Histograms are built by incrementing a counter in a bucket and stored as a sparse list of 
+bucket -> count entries.  The bucket is defined by the smallest number stored in that bucket, computed
+by using an IEEE Double with a mantissa truncated to n bits of precision.  The default in MAD is to use
+7 bits of precision.  Additionally, the min, max and sum of the samples are stored as IEEE Double values
+alongside the histogram.
+
+Statistics
+----------
+
+min - accurate (stored)  
+max - accurate (stored)  
+sum - accurate (stored)  
+quantiles - estimated.  accuracy depends on precision of the histogram and value stored.  n bits of 
+precision provides accuracy to within (1 / (2^n)) * value.  ex: 7 bits of precision provides accuracy 
+to within 1% of the computed value.
+