add a link to the command docs

Thanks to Francis Lavoie for pointing this out in #6.

Author: mvdan

README.md

@@ -177,8 +177,9 @@ The `setup-go` action doesn't set `PATH`, so currently it's not possible to `go
 install` a program and run it directly. Until that's fixed, consider absolute
 paths like `$(go env GOPATH)/bin/program`.
 
-An undocumented workaround is to use a built-in special string to add `GOBIN` to
-the system's `PATH`. Note that `shell: bash` is supported on all platforms:
+A workaround is to use a built-in [workflow command](https://help.github.com/en/actions/reference/workflow-commands-for-github-actions#adding-a-system-path)
+to add `GOBIN` to the system's `PATH`. Note that `shell: bash` is supported on
+all platforms:
 
 ```yaml
     - name: Add GOBIN to PATH