Correct & clarify release instructions

Co-authored-by: Ed Morley <501702+edmorley@users.noreply.github.com>
Signed-off-by: Mars Hall <mars@users.noreply.github.com>

Author: mars

README.md

@@ -131,7 +131,7 @@ _Replace "X.X" with the new version number._
 1. Update `CHANGELOG.md`, moving the **[unreleased] - YYYY-MM-DD** section contents into a new version section for version X.X
 2. Create a branch `preparing-release-X.X` with PR **Preparing release X.X**
 3. Get the PR approved
-4. Create a new [GitHub release](https://github.com/heroku/heroku-buildpack-nginx/releases) for the new version
-5. Merge the approved PR
-6. [Publish a new release](https://addons-next.heroku.com/buildpacks/cf2713a5-65d0-4bc8-8dd2-b00d8d4f03f4/publish) in Heroku Buildpack Registry.
+4. Merge the approved PR
+5. Create a new [GitHub release](https://github.com/heroku/heroku-buildpack-nginx/releases) for the new version, targetting `main`, with name and tag of form `vX.Y`.
+6. Publish a new release in the Heroku Buildpack Registry, using either [plugin-buildpack-registry](https://github.com/heroku/plugin-buildpack-registry) and `heroku buildpacks:publish heroku-community/nginx <git_tag>`, or else the [buildpacks dashboard](https://addons-next.heroku.com/buildpacks/cf2713a5-65d0-4bc8-8dd2-b00d8d4f03f4/publish).