add package godoc

So that pkg.go.dev and `go doc` have something to show.
Copying the first sentence from the README for brevity.

main.go

@@ -1,6 +1,7 @@
 // Copyright (c) 2019, The Garble Authors.
 // See LICENSE for licensing information.
 
+// garble obfuscates Go code by wrapping the Go toolchain.
 package main
 
 import (