Add releasing documentation

Document how maintainers perform releases.

Author: MrAlias

RELEASING.md

@@ -0,0 +1,90 @@
+# Release Process
+
+## Pre-Release
+
+First, decide which module sets will be released and update their versions in `versions.yaml`.
+Commit this change to a new branch (i.e. `release-vX.X.X`).
+
+Update all crosslink dependencies and any version references in code.
+
+1. Run the `prerelease` make target.
+
+   ```console
+   make prerelease MODSET=<module set>
+   ```
+
+   For example, to prepare a release for the `obi` module set, run:
+
+   ```console
+   make prerelease MODSET=obi
+   ```
+
+   This will create a branch `prerelease_<module set>_<new tag>` that will contain all release changes.
+
+2. Verify the changes.
+
+    ```console
+    git diff ...prerelease_<module set>_<new tag>
+    ```
+
+    This should have changed the version for all modules to be `<new tag>`, if there are any crosslink dependencies.
+
+    If these changes look correct, merge them into your pre-release branch:
+
+    ```console
+    git merge prerelease_<module set>_<new tag>
+    ```
+
+3. Push the changes to upstream and create a Pull Request on GitHub.
+   Be sure to include the curated changes from the [Changelog](./CHANGELOG.md) in the description.
+
+## Tag
+
+Once the Pull Request with all the version changes has been approved and merged it is time to tag the merged commit.
+
+<!-- markdownlint-disable MD028 -->
+> [!CAUTION]
+> It is critical you use the same tag that you used in the Pre-Release step!
+> Failure to do so will leave things in a broken state.
+> As long as you do not change `versions.yaml` between pre-release and this step, things should be fine.
+
+> [!CAUTION]
+> [There is currently no way to remove an incorrectly tagged version of a Go module](https://github.com/golang/go/issues/34189).
+> It is critical you make sure the version you push upstream is correct.
+> [Failure to do so will lead to minor emergencies and tough to work around](https://github.com/open-telemetry/opentelemetry-go/issues/331).
+<!-- markdownlint-enable MD028 -->
+
+1. For each module set that will be released, run the `add-tags` make target using the `<commit-hash>` of the commit on the main branch for the merged Pull Request.
+
+   ```console
+   make add-tags MODSET=<module set> COMMIT=<commit hash>
+   ```
+
+   For example, to add tags for the `obi` module set for the latest commit, run:
+
+   ```console
+   make add-tags MODSET=obi
+   ```
+
+   It should only be necessary to provide an explicit `COMMIT` value if the
+   current `HEAD` of your working directory is not the correct commit.
+
+2. Push tags to the upstream remote (not your fork: `github.com/open-telemetry/opentelemetry-go.git`).
+   Make sure you push all sub-modules as well.
+
+   ```console
+   git push upstream <new tag>
+   git push upstream <submodules-path/new tag>
+   ...
+   ```
+
+## Release
+
+Finally create a Release for the new `<new tag>` on GitHub.
+
+Currently we do not have a curated changelog.
+Use the Github automated changelog generation to create the release notes.
+
+## Post-Release
+
+**TODO**: bump versions in Helm charts and other places.