Add `cl_logic` folder to README.

README.md

@@ -100,6 +100,9 @@ followed by `make build-dep`.
   modules; they may change or disappear without any notice.
 * The folder [si_logic](theories/si_logic) defines a "plain" step-indexed logic
   and shows that it is an instance of the BI interface.
+* The folder [cl_logic](theories/cl_logic) embeds classical logic in Coq,
+  following essentially the Gödel-Gentzen translation, and shows that it is an
+  instance of the BI interface.
 
 ## Case Studies