add note about revisions

Author: dsp-ant

docs/specification/revisions/_index.md

@@ -3,3 +3,6 @@ title: Revisions
 cascade:
   type: docs
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+The Model Context Protocol uses revisions that define specific protocol behaviors for each version. Each protocol revision provides guarantees about the functionality and semantics available. The protocol version is exchanged during initialization (see {{ < relref "../basic/versioning.md" > }}).
+
+Revisions can be marked as either DRAFT (in-progress specifications) or FINAL (completed specifications). DRAFT revisions may change at any time, while FINAL revisions remain stable once published.