doc: add commit formats for release blog posts

this commit enhances the guidelines to creating a release blog post, specifically by adding
a commit format that must be adhered to when creating a pull request on the website repository

Fixes: https://github.com/nodejs/node/issues/8629

PR-URL: https://github.com/nodejs/node/pull/8631
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: James M Snell <jasnell@keybase.io>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>

doc/releases.md

@@ -294,7 +294,12 @@ Create a new blog post by running the [nodejs.org release-post.js script](https:
 * You can add a short blurb just under the main heading if you want to say something important, otherwise the text should be publication ready.
 * The links to the download files won't be complete unless you waited for the ARMv6 builds. Any downloads that are missing will have `*Coming soon*` next to them. It's your responsibility to manually update these later when you have the outstanding builds.
 * The SHASUMS256.txt.asc content is at the bottom of the post. When you update the list of tarballs you'll need to copy/paste the new contents of this file to reflect those changes.
-* Always use pull-requests on the nodejs.org repo. Be respectful of that working group, but you shouldn't have to wait for PR sign-off. Opening a PR and merging it immediately _should_ be fine.
+* Always use pull-requests on the nodejs.org repo. Be respectful of that working group, but you shouldn't have to wait for PR sign-off. Opening a PR and merging it immediately _should_ be fine. However, please follow the following commit message format:
+```
+Blog: vX.Y.Z release post
+
+Refs: <full URL to your release proposal PR>
+```
 * Changes to `master` on the nodejs.org repo will trigger a new build of nodejs.org so your changes should appear in a few minutes after pushing.
 
 ### 15. Announce