add RELEASE.md document

Sources/Prometheus/RELEASE.md

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+# Releasing 
+
+## Release schedule
+
+Releases of this library are performed on a when-needed basis. 
+
+Generally one of the maintainers listed in MAINTAINERS.md will perform a release.
+
+If you would like to request a release since a change or fix has been waiting for a release for some time,
+you may open up an issue on github, requesting a release.
+
+## Versioning 
+
+As usual with Swift packages, we use [Semantic Versioning](https://semver.org/).
+
+## Make a release
+
+Development and releases are made from the `main` branch directly, therefore the branch should remain in releasable state at any time.
+
+Making a release is simple and boils down to making a git tag and pushing it to the upstream repository.
+
+Tags should follow the simple `MAJOR.MINOR.PATCH` format, without any prefix (e.g. do not include the `v` prefix).
+
+Step 1) Make a release tag.
+
+```bash
+tag="MAJOR.MINOR.PATCH"
+git tag -s "${tag}" -m "${tag}"
+git push origin "${tag}"
+```
+
+Step 2) Make the GitHub Release and Release Notes.
+
+Release notes generation is automated and based on the enforced labelling of pull requests.
+As pull requests are merged, they are required to be tagged with `semver/*` tags (`semver/patch`, `semver/minor`, ...).
+When you navigate to the [New release](https://github.com/swift-server/swift-prometheus/releases/new) page on github and 
+select your release tag, and click **Generate release notes** release notes will automatically be generated.
+
+Take a moment to read over the changes and make sure their assigned patch/minor/major categories are correct and match the release's assigned version.
+
+You may add some additional introduction, thanks or additional information which may be useful for anyone reading the release notes.
+
+Step 3) "There is no Step 3!"
+
+## Swift Versions
+
+Generally this project supports three most recent minor versions of Swift.
+
+For example, at time of writing this document Swift 6.2 is the current release, so this project also supports 6.1 and 6.0.
+And this list of versions changes whenever a new minor Swift version is released.