update histogram doc

Author: BrandonArp

Histograms.md

@@ -12,12 +12,20 @@ by using an IEEE Double with a mantissa truncated to n bits of precision.  The d
 7 bits of precision.  Additionally, the min, max and sum of the samples are stored as IEEE Double values
 alongside the histogram.
 
+For a given value, the bucket is found by the following:  
+bucket = (double)(val &  0xffffe00000000000L)
+
+The result is a consistent set of buckets that scale with the value of the data.  Larger values
+have a larger absolute error, but with a consistent error proportion.
+
 Statistics
 ----------
 
-min - accurate (stored)
-max - accurate (stored)
-sum - accurate (stored)
+min - accurate (stored)  
+max - accurate (stored)  
+sum - accurate (stored)  
+count - accurate (derived)  
+mean - accurate (derived)  
 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.