Add a note stating that underlying IETF pieces are still in flux (#120)

* Add a note stating that underlying IETF pieces are still in flux

* Moving note

* Remove coma

Author: youennf

index.bs

@@ -172,6 +172,13 @@ Similarly, if a web application sets the transform synchronously at creation of
 
 # SFrameTransform # {#sframe}
 
+<p class="note">
+The API presented in this section represents a preliminary proposal
+based on protocol proposals that have not yet been adopted by an IETF WG.
+As a result, both the API and underlying protocol are likely to change
+significantly going forward.
+</p>
+
 <xmp class="idl">
 enum SFrameTransformRole {
     "encrypt",