try to add duplicate warning definition only for knitr

Author: cderv

src/resources/schema/cell-textoutput.yml

@@ -69,6 +69,19 @@
 
       Note that this option is supported only for the `revealjs` format.
 
+- name: warning
+  tags:
+    engine: knitr
+  schema:
+    enum: [true, false, NA]
+  default: true
+  description:
+    short: Include warning in rendered output.
+    long: |
+      Include warnings in rendered output. Possible values are `true`, `false`, or `NA`. 
+      If `true`, messages are included in the output. If `false`, messages are not included. 
+      If `NA`, messages are not included in output but shown in the knitr log to console.
+
 - name: message
   tags:
     engine: knitr