document the CI a little

.github/workflows/flambda-backend.yml

@@ -3,6 +3,17 @@ on:
   push:
   pull_request:
 
+# Overall approach to running the tests:
+#  - Install OCaml 5.1, as this is the version of dependencies our merlin expects.
+#  - Build flambda-backend (which requires installing OCaml 4.14).
+#  - Build merlin using OCaml 5.1, and then run the tests while pointing merlin at
+#    flambda-backend's stdlib and ocamlc.
+#
+# The second step only needs to be run when we update the flambda-backend version,
+# so we cache it. We use [upstream/ocaml_flambda/base-rev.txt] as the cache key,
+# given that it is updated when and only when we import new changes from
+# flambda-backend into merlin-jst.
+
 jobs:
   build:
     # The type of runner that the job will run on