regexp: add simple package-level example

Update #4125

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/6846045

src/pkg/regexp/example_test.go

+package regexp_test
+
+import (
+	"fmt"
+	"regexp"
+)
+
+func Example() {
+	// Compile the expression once, usually at init time.
+	// Use raw strings to avoid having to quote the backslashes.
+	var validID = regexp.MustCompile(`^[a-z]+\[[0-9]+\]$`)
+
+	fmt.Println(validID.MatchString("adam[23]"))
+	fmt.Println(validID.MatchString("eve[7]"))
+	fmt.Println(validID.MatchString("Job[48]"))
+	fmt.Println(validID.MatchString("snakey"))
+	// Output:
+	// true
+	// true
+	// false
+	// false
+}