Add SOP on how to create a new release (#404)

scripts/README.md

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+# Creating a release
+
+SOP to create a new release in smtml.
+
+## Semi-automatically
+
+1. Trigger the [Release PR workflow].
+2. Approve the PR and merge changes to **main**.
+3. Locally, pull the changes and tag the latest commit with `v[VERSION]`, where `VERSION` is the version created by the workflow.
+4. Push the tag.
+4. Re-run the latest [opam-repository CI] to automatically publish the package.
+5. Track the created PR in the opam-repository to check for CI errors or comments from maintainers.
+
+**Note**: There is a cron job in GitHub that is set to run the [Release PR workflow] every monday at 9AM.
+
+[Release PR workflow]: https://github.com/formalsec/smtml/actions/workflows/release.yml
+[opam-repository CI]: https://github.com/formalsec/smtml/actions/workflows/publish.yml
+
+## Manually
+
+```sh
+# 1. Run the create-release.sh script:
+#   a. If no version is specified, the create-release.sh script simply bumps the current version.
+./create-release.sh
+#   b. If specified, it will use the provided tag instead. Example:
+# ./create-release.sh 0.18.0
+
+# 2. Approve the created PR and merge it to main.
+
+# 3. Update the main branch:
+git switch main
+git fetch --all
+git rebase
+
+# 4. Create the tag:
+git tag -a vVERSION # Where VERSION is the desired version
+git push -u origin vVERSION
+
+# 5. Publish new version to the opam-repository:
+./publish.sh
+```