Add CHANGELOG.md and update CONTRIBUTING.md (#13673)

* Add github action requiring a diff to changelog.md.

* Update CONTRIBUTING.md with CHANGELOG.md requirements and restrictions on trivial PRs

* Update tj-actions/changed-files

* Backfill CHANGELOG.md

* Remove hashtag references to PRs

* Append PR template

* Add unreleased changes

* Update CONTRIBUTING.md

Co-authored-by: Radosław Kapka <rkapka@wp.pl>

---------

Co-authored-by: Radosław Kapka <rkapka@wp.pl>

.github/PULL_REQUEST_TEMPLATE.md

@@ -10,6 +10,7 @@
    in review.
 4. Note that PRs updating dependencies and new Go versions are not accepted.
    Please file an issue instead.
+5. A changelog entry is required for user facing issues.
 -->
 
 **What type of PR is this?**
@@ -28,3 +29,9 @@
 Fixes #
 
 **Other notes for review**
+
+**Acknowledgements**
+
+- [ ] I have read [CONTRIBUTING.md](https://github.com/prysmaticlabs/prysm/blob/develop/CONTRIBUTING.md).
+- [ ] I have made an appropriate entry to [CHANGELOG.md](https://github.com/prysmaticlabs/prysm/blob/develop/CHANGELOG.md).
+- [ ] I have added a description to this PR with sufficient context for reviewers to understand this PR.

.github/workflows/changelog.yml

@@ -0,0 +1,33 @@
+name: CI
+
+on:
+  pull_request:
+    branches:
+      - develop
+
+jobs:
+  changed_files:
+    runs-on: ubuntu-latest 
+    name: Check CHANGELOG.md
+    steps:
+      - uses: actions/checkout@v4
+      - name: changelog modified
+        id: changelog-modified
+        uses: tj-actions/changed-files@v45
+        with:
+          files: CHANGELOG.md
+
+      - name: List all changed files
+        env:
+          ALL_CHANGED_FILES: ${{ steps.changelog-modified.outputs.all_changed_files }}
+        run: |
+          if [[ ${ALL_CHANGED_FILES[*]} =~ (^|[[:space:]])"CHANGELOG.md"($|[[:space:]]) ]];
+          then
+            echo "CHANGELOG.md was modified.";
+            exit 0;
+          else
+            echo "CHANGELOG.md was not modified.";
+            echo "Please see CHANGELOG.md and follow the instructions to add your changes to that file."
+            echo "In some rare scenarios, a changelog entry is not required and this CI check can be ignored."
+            exit 1;
+          fi