Add badge link to Go API docs in pkg.go.dev (google#676)

Thought this might be useful to point people at the API docs (after I
tried searching for pprof API docs and couldn't find them easily on
Google).

Points at https://pkg.go.dev/github.com/google/pprof/profile, as that's
the only package we try to declare as an API.

Badge generated at
https://pkg.go.dev/badge/?path=https%3A%2F%2Fpkg.go.dev%2Fgithub.com%2Fgoogle%2Fpprof%2Fprofile

Author: mhansen

README.md

@@ -1,5 +1,6 @@
 [![Github Action CI](https://github.com/google/pprof/workflows/ci/badge.svg)](https://github.com/google/pprof/actions)
 [![Codecov](https://codecov.io/gh/google/pprof/graph/badge.svg)](https://codecov.io/gh/google/pprof)
+[![Go Reference](https://pkg.go.dev/badge/github.com/google/pprof/profile.svg)](https://pkg.go.dev/github.com/google/pprof/profile)
 
 # Introduction