Add changelog check to CI (#1104)

* Add changelog check to CI

* Add changelog note to CONTRIBUTING.md

* Bump changelog-check-action to v2

* fmt

.github/workflows/changelog.yml

@@ -0,0 +1,13 @@
+name: Changelog check
+
+on:
+  pull_request:
+    branches: [master]
+    types: [opened, synchronize, reopened, labeled, unlabeled]
+
+jobs:
+  Changelog-Entry-Check:
+    name: Check Changelog Action
+    runs-on: ubuntu-20.04
+    steps:
+      - uses: tarides/changelog-check-action@v2

CONTRIBUTING.md

@@ -73,6 +73,13 @@ very useful to:
 - Run the tests: this will check that all the data is correctly formatted and
   can be invoked with `make test`
 
+### Changelog
+
+User-visible changes should come with an entry in the changelog under the
+appropriate part of the unreleased section. PR that doesn't provide an entry
+will fail CI check. This behavior can be overridden by using the "no changelog"
+label, which is used for changes that are not user-visible.
+
 ## Repository Structure
 
 The following snippet describes VSCode OCaml's repository structure.