Add doc.go file to show description and usage on godoc.org

doc.go

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+/*
+Package stats is a well tested and comprehensive
+statistics library package with no dependencies.
+
+Example Usage:
+
+	// start with some source data to use
+	data := []float64{1.0, 2.1, 3.2, 4.823, 4.1, 5.8}
+
+	// you could also use different types like this
+	// data := stats.LoadRawData([]int{1, 2, 3, 4, 5})
+	// data := stats.LoadRawData([]interface{}{1.1, "2", 3})
+	// etc...
+
+	median, _ := stats.Median(data)
+	fmt.Println(median) // 3.65
+
+	roundedMedian, _ := stats.Round(median, 0)
+	fmt.Println(roundedMedian) // 4
+
+MIT License Copyright (c) 2014-2019 Montana Flynn
+*/
+package stats