bind: mention the supported binding rules.

Change-Id: I4760042871b2ccb7b24ee79ef9adef54cfb9303b
Reviewed-on: https://go-review.googlesource.com/9996
Reviewed-by: Alan Donovan <adonovan@google.com>

bind/bind.go

@@ -4,7 +4,9 @@
 
 // Package bind implements a code generator for gobind.
 //
-// See the documentation on the gobind command for usage details.
+// See the documentation on the gobind command for usage details
+// and the list of currently supported types.
+// (http://godoc.org/golang.org/x/mobile/cmd/gobind)
 package bind // import "golang.org/x/mobile/bind"
 
 // TODO(crawshaw): slice support