README.md: add badge to pkg.go.dev

Change-Id: Ib78795f29fae39020392c84b00e6e4e26b7cd9de
Reviewed-on: https://go-review.googlesource.com/c/perf/+/275937
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
diff --git a/README.md b/README.md
index f354c3a..4691e5b 100644
--- a/README.md
+++ b/README.md
@@ -1,5 +1,7 @@
 # Go performance measurement, storage, and analysis tools
 
+[![Go Reference](https://pkg.go.dev/badge/golang.org/x/perf.svg)](https://pkg.go.dev/golang.org/x/perf)
+
 This subrepository holds the source for various packages and tools
 related to performance measurement, storage, and analysis.