document Coq version support policy

README.md

@@ -33,6 +33,8 @@ This version is known to compile with:
  - Coq 8.13.2 / 8.14.1 / 8.15.2 / 8.16.1
  - A development version of [std++](https://gitlab.mpi-sws.org/iris/stdpp)
 
+Generally we always aim to support at least the last two stable Coq releases.
+
 If you need to work with older versions of Coq, you can check out the
 [tags](https://gitlab.mpi-sws.org/iris/iris/-/tags) for old Iris releases that
 still support them.