githubci: add a workflow for creating GitHub release notes

GitHub now allows users to subscribe only to
"release" notifications of a repository.
So, users can be notified of new releases and their
changelog/release notes automatically.

This workflow works whenever:
    a new version tag
    (with the format following the regex "v\d+\..*")
    is pushed to the repository
AND
    the commit that the tag points to, created/modified
    a release notes file from Doumentation/RelNotes/ directory.

The script for generating the temporary changelog file is
written in Kotlin language which can be a much better alternative
to shell scripts in terms of features and readability
(it is like a python script but with static typing support).
The Kotlin runner is pre-installed in GitHub Actions environments;
for more information see
    https://github.com/actions/virtual-environments/
    https://stackoverflow.com/a/69116750/8583692

The "Release Notes (yyyy-MM-dd)" link in https://git-scm.com/
website can also link to these GitHub release pages instead of
to the raw .txt release note file in the repository.

See the issue related to GitHub release notifications:
https://github.com/isaacs/github/issues/410

Also see GitHub announcement for this feature:
https://github.blog/changelog/2018-11-27-watch-releases/

Signed-off-by: Mahdi Hosseinzadeh <mdihosseinzadeh@gmail.com>

.github/scripts/generate-github-changelog.main.kts

@@ -0,0 +1,21 @@
+#!/usr/bin/env kotlin
+
+/**
+ * Copies contents of the release notes file created/modified
+ * in this commit to a new file to be used by the workflow.
+ */
+
+import java.io.File
+
+println("Files modified in this commit:")
+args.forEachIndexed { index, name ->
+    println("\t${index + 1}- $name")
+}
+
+val notesFile = args
+    .map(::File)
+    .singleOrNull { "RelNotes" in it.parent }
+
+notesFile
+    ?.copyTo(File("changelog.txt"))
+    ?: println("No release notes file modified in this commit")

.github/workflows/create-release.yml

@@ -0,0 +1,40 @@
+name: Create GH release
+
+# Create a GitHub release for each new tag.
+# The release notes are taken from the release notes file
+# modified in that commit located in Documentation/RelNotes directory.
+
+on:
+  push:
+    tags:
+      - v[0-9]+.*
+
+permissions:
+  contents: write
+
+jobs:
+  create-gh-release:
+    name: Create a new release or update an existing release in the GitHub repository
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout the repository
+        uses: actions/checkout@v2
+        with:
+          fetch-depth: 2  # OR '0' To retrieve all preceding commit.
+      - name: Get changed files
+        uses: tj-actions/changed-files@v11.7
+        id: changed-files
+        with:
+          separator: ' '
+      - name: Generate the changelog
+        run: kotlin .github/scripts/generate-github-changelog.main.kts ${{ steps.changed-files.outputs.all_changed_and_modified_files }}
+      - name: Create the release
+        uses: actions/create-release@v1
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        with:
+          tag_name: ${{ github.ref_name }}
+          release_name: ${{ github.ref_name }}
+          body_path: changelog.txt
+          draft: false
+          prerelease: false