cmd/benchstat: add examples to README for populating old.txt and new.txt

benchcmp had similar examples and I found them easier than remembering
the exact commands to run.

https://pkg.go.dev/golang.org/x/tools/cmd/benchcmp

Change-Id: I539e34774d1f09762c164e633680ee1e7157499d
Reviewed-on: https://go-review.googlesource.com/c/perf/+/429555
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
diff --git a/cmd/benchstat/README.md b/cmd/benchstat/README.md
index 0a299af..db5ec61 100644
--- a/cmd/benchstat/README.md
+++ b/cmd/benchstat/README.md
@@ -37,8 +37,14 @@
 
 ## Example
 
-Suppose we collect benchmark results from running `go test -bench=Encode`
-five times before and after a particular change.
+Suppose we collect results from running a set of benchmarks twenty times
+before a particular change:
+
+    go test -run='^$' -bench=. -count=20 > old.txt
+
+And the same benchmarks twenty times after:
+
+    go test -run='^$' -bench=. -count=20 > new.txt
 
 The file old.txt contains:
 
diff --git a/cmd/benchstat/main.go b/cmd/benchstat/main.go
index 84f3849..071d6b4 100644
--- a/cmd/benchstat/main.go
+++ b/cmd/benchstat/main.go
@@ -43,8 +43,14 @@
 //
 // # Example
 //
-// Suppose we collect benchmark results from running “go test -bench=Encode”
-// five times before and after a particular change.
+// Suppose we collect results from running a set of benchmarks twenty times
+// before a particular change:
+//
+//	go test -run='^$' -bench=. -count=20 > old.txt
+//
+// And the same benchmarks twenty times after:
+//
+//	go test -run='^$' -bench=. -count=20 > new.txt
 //
 // The file old.txt contains:
 //