user guide: rename Reference -> Appendix

add YARD API Documentation link

doc/user_guide.md

@@ -993,7 +993,7 @@ Example:
 ${include build_tests/json_to_yaml/Rsconscript}
 ```
 
-#> Reference
+#> Appendix
 
 ## Default Construction Variables
 
@@ -1104,6 +1104,10 @@ build do
 end
 ```
 
+##> YARD API Documentation
+
+See [here](../yard/index.html) for Rscons YARD API Documentation.
+
 #> License
 
 Rscons is licensed under the terms of the MIT License: