Update witx.md (#263)

clarify .witx is not going away

Author: benbrittain

docs/witx.md

@@ -17,10 +17,10 @@ full interface types specification, or the use of the interface types custom
 sections.
 
 We expect that eventually we will transition to using the full interface
-types specification. Until then, the goals here are to remain aligned with
-interface types and other relevant WebAssembly standards and proposals
-wherever practical, and to be an input into the design process of interface
-types.
+types specification, with `witx` having minimal additional features. Until then,
+the goals here are to remain aligned with interface types and other relevant
+WebAssembly standards and proposals wherever practical, and to be an input 
+into the design process of interface types.
 
 [module types]: https://github.com/WebAssembly/module-types/blob/master/proposals/module-types/Overview.md
 [interface types]: https://github.com/WebAssembly/interface-types/blob/master/proposals/interface-types/Explainer.md