add CI file

.gitlab-ci.yml

+image: coq:8.5
+
+stages:
+  - iris
+  - iris-atomic
+
+iris:
+  stage: iris
+  tags:
+  - coq
+  script:
+  - coqc -v
+  # see if the Iris submodule needs cleaning, then build it
+  - 'git submodule status iris | egrep "^ " || (git submodule update --init iris && cd iris && git clean -xfd)'
+  - 'cd iris && make -j8'
+  only:
+  - master
+  - ci
+
+iris-atomic:
+  stage: iris-atomic
+  tags:
+  - coq
+  script:
+  - coqc -v
+  # prepare the environment, safeguard against outdated submodule
+  - 'git submodule status iris | egrep "^ "'
+  - 'ln -s iris iris-enabled'
+  # build local repo
+  - 'time make -j8'
+  only:
+  - master
+  - ci