[spec] add roots to list of client features

Author: dsp-ant

docs/specification/draft/index.mdx

@@ -64,6 +64,7 @@ Servers offer any of the following features to clients:
 Clients may offer the following features to servers:
 
 - **Sampling**: Server-initiated agentic behaviors and recursive LLM interactions
+- **Roots**: Server-initiated inquiries into uri or filesystem boundaries to operate in
 - **Elicitation**: Server-initiated requests for additional information from users
 
 ### Additional Utilities