readme: add pkg.go.dev badge (#46)

Since the go.mod uses a custom URL, it's easier to provide the badge for
new users to direct to the right URL.
diff --git a/README.md b/README.md
index ceff09b..69026e0 100644
--- a/README.md
+++ b/README.md
@@ -31,6 +31,8 @@
 
 ## Development
 
+[![Go Reference](https://pkg.go.dev/badge/sketch.dev.svg)](https://pkg.go.dev/sketch.dev)
+
 See [CONTRIBUTING.md](CONTRIBUTING.md)
 
 ## Open Source