Better documentation

Author: Unitech

HISTOGRAM.md

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+#### Options
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+**measurement** option can be:
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+- min: The lowest observed value.
+- max: The highest observed value.
+- sum: The sum of all observed values.
+- variance: The variance of all observed values.
+- mean: The average of all observed values.
+- stddev: The stddev of all observed values.
+- count: The number of observed values.
+- median: 50% of all values in the resevoir are at or below this value.
+- p75: See median, 75% percentile.
+- p95: See median, 95% percentile.
+- p99: See median, 99% percentile.
+- p999: See median, 99.9% percentile.