Add open-uri reference

Author: Bill Desmarais

README.md

@@ -20,11 +20,12 @@ build tool.  The syntax is Rake-like, so it should be familiar to most Rake
 users.
 
 Please note: Thor, by design, is a system tool created to allow seamless file and url
-access, which should not receive application user input. It relies on open-uri,
+access, which should not receive application user input. It relies on [open-uri][open-uri],
 which combined with application user input would provide a command injection attack
 vector.
 
 [rake]: https://github.com/ruby/rake
+[open-uri]: https://ruby-doc.org/stdlib-2.5.1/libdoc/open-uri/rdoc/index.html
 
 Installation
 ------------