doc.go: revise text, add performance

doc.go

@@ -3,9 +3,12 @@ Package hdrhist provides high dynamic range (HDR) histograms.
 
 HDR histograms can be used to accurately record and analyze
 distributions with very large ranges of data.
-HDR histograms are compact: with a precision of 3 significant digits
-and a range of 1000-100 billion (e.g. 1 μs to 100 s),
-an HDR histogram consumes about 156 KB.
+This HDR histogram implementation is meant to have minimal memory usage
+and support recording values quickly.
+With a precision of 3 significant digits and a range of 1000-100 billion
+(e.g. 1 μs to 100 s), an HDR histogram consumes about 156 KB.
+Benchmarks show that recording a value in a histogram should take about 12 ns
+depending on the hardware.
 
 A typical usecase for HDR Histograms would be recording latency values
 in client or server software.