docs: Add v2.0.0 release notes

Add v2.0.0 release notes to the docs/release-notes directory. This will
trigger a new major release.

docs/release-notes/v2.0.0.md

@@ -0,0 +1,24 @@
+## API Changes and Discord Integration
+
+**Breaking change**: the previous input parameter `SLACK_TOKEN` and `CHANNEL`
+have been deprecated and cannot be used anymore. Use `SLACK_WEBHOOK_URL` and
+`SLACK_CHANNEL` instead.
+
+`howl` can now be used as GitHub Action with `with` inputs:
+
+```yaml
+  howl-on-failure:
+    steps:
+      - uses: foxygoat/howl@v2
+        with:
+          slack_webhook_url: ${{ secrets.SLACK_WEBHOOK_URL }}
+          slack_channel: C0000000000 # optional; channel ID
+          slack_text: <!here> # optional; text or @-mention
+          discord_webhook_url: ${{ secrets.DISCORD_WEBHOOK_URL }}
+          discord_text: @here # optional; text or @-mention
+```
+
+Discord integration has been added.
+
+GitHub Releases are now created on every merge to `master` distributing the
+`howl` script to be used with hermit.