add reference to uris

Author: kentcdodds

exercises/03.resources/README.mdx

@@ -63,6 +63,10 @@ jira://myworkspace/ticket/304
 amazon://product/1234567890
 ```
 
+📜 If you're unfamiliar with URIs and their formats, you can read more about
+them on Wikipedia:
+[Uniform Resource Identifier](https://en.wikipedia.org/wiki/Uniform_Resource_Identifier).
+
 MCP resources are designed to be application-driven: you decide what to expose,
 and how. Clients can list available resources, read their contents, and
 (optionally) subscribe to updates or changes. The protocol supports both static