Mention GitHub URL in the API documentation.

yaml.go

diff --git a/yaml.go b/yaml.go
index 4944dac..bc5cc73 100644
--- a/yaml.go
+++ b/yaml.go
@@ -1,4 +1,9 @@
 // Package yaml implements YAML support for the Go language.
+//
+// Source code and other details for the project are available at GitHub:
+//
+//   https://github.com/go-yaml/go-yaml-v1
+//
 package yaml
 
 import (