README: add a godoc badge

If someone is new to the repository, it might not be obvious that there
is a Go API available as a library. The README does later refer to the
format library, but also add it to the top as a badge like any other library.

Author: mvdan

README.md

@@ -1,5 +1,7 @@
 # gofumpt
 
+[![Go Reference](https://pkg.go.dev/badge/mvdan.cc/gofumpt/format.svg)](https://pkg.go.dev/mvdan.cc/gofumpt/format)
+
 	go install mvdan.cc/gofumpt@latest
 
 Enforce a stricter format than `gofmt`, while being backwards compatible.