gitlab-ci: add a comment describing what goes into a job

.gitlab-ci.yml

@@ -14,6 +14,22 @@ stages:
     - test
     - test-ext
 
+################################################################################
+# Job declarations
+#
+# Each job must pull in each of the following keys:
+#
+#   - a "base image"
+#   - a build script
+#   - tags for the jobs
+#   - rules for when to run the job
+#
+# Additionally, jobs may also contain:
+#
+#   - artifacts
+#   - dependency/needs jobs for required jobs
+################################################################################
+
 # Lint builds
 
 build:debian10-iwyu: