Update README.

README.md

@@ -40,6 +40,9 @@ running:
     language, e.g., parallel composition.
     Most notable here s `lib/barrier`, the implementation and proof of a barrier
     as described in <http://doi.acm.org/10.1145/2818638>.
+* The folder `proofmode` contains the Iris proof mode, which extends Coq with
+  contexts for persistent and spatial Iris assertions. It also contains tactics
+  for interactive proofs in Iris.
 * The folder `tests` contains modules we use to test our infrastructure.
   Users of the Iris Coq library should *not* depend on these modules; they may
   change or disappear without any notice.