update README to reflect the branch's state

.gitlab-ci.yml

-image: ralfjung/opam-ci:latest
-
-iris-coq8.5.3:
-  tags:
-  - fp-timing
-  script:
-  # prepare
-  - . build/opam-ci.sh 'coq 8.5.3' 'coq-mathcomp-ssreflect 1.6.1'
-  # build
-  - 'time make -j8 TIMED=y 2>&1 | tee build-log.txt'
-  - 'if fgrep Axiom build-log.txt >/dev/null; then exit 1; fi'
-  - 'cat build-log.txt | egrep "[a-zA-Z0-9_/-]+ \(user: [0-9]" | tee build-time.txt'
-  - 'if (( RANDOM % 10 == 0 )); then make validate; fi'
-  cache:
-    key: "coq8.5.3"
-    paths:
-    - opamroot/
-  only:
-  - iris-3.0
-  artifacts:
-    paths:
-    - build-time.txt
-
-iris-coq8.6:
-  tags:
-  - fp-timing
-  script:
-  # prepare
-  - . build/opam-ci.sh 'coq 8.6' 'coq-mathcomp-ssreflect 1.6.1'
-  # build
-  - 'time make -j8'
-  cache:
-    key: "coq8.6"
-    paths:
-    - opamroot/
-  only:
-  - iris-3.0

README.md

-# IRIS COQ DEVELOPMENT
+# IRIS COQ DEVELOPMENT (3.0 branch)
 
-This is the Coq development of the [Iris Project](http://iris-project.org).
+This is the 3.0 branch of the Coq development of the
+[Iris Project](http://iris-project.org).  This branch is unmaintained.  For the
+latest version,
+[please switch to the master branch](https://gitlab.mpi-sws.org/iris/iris/).
 
 ## Prerequisites