Add a readme to the ci directory

ci/README.md

+# What's here?
+This folder contains the scripts run by the CI pipeline on GitLab.
+The [`.gitlab-ci.yml`](.gitlab-ci.yml) at the top level of the repo
+specifies the two stages of the pipeline, __Build__ and __Test__.
+
+## Build Stage
+The build stage is defined by the [`ci-build.sh`](ci/ci-build.sh) file.
+It runs [make](quad/Makefile) under the quad directory to build the project.
+
+## Test Stage
+The test stage is defined by the [`ci-test.sh`](ci/ci-test.sh) file.
+It runs [make test](quad/Makefile#36) under the quad directory to run the tests.