added pre version of a wiki

wiki/dev/rnk.md

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+# Roll n Keep (RnK)
+The RnK use `ChatMessage` to retrieve the roll, alter it, add the new message and delete the old.
+
+> If you have any idea how to modify directly the ChatMessage and update it, let me know.
+
+Usage :
+```js
+new RollnKeepDialog(messageId).render(true);
+```