Add documentation of release process

RELEASE.md

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+# Releasing connect-kotlin
+
+Connect-Kotlin’s repository has a release workflow which publishes connect-kotlin
+artifacts to public maven. The release process was intentionally designed to be
+as friction-free as possible — the result is that you just create/tag a release
+in GitHub, and everything else is automated after that.
+
+Using the Github UI, create a new release like so:
+* Under “Choose a tag”, type in “vX.Y.Z” to create a new tag for the release upon publish.
+  Note: The release job does infer the version from the release string and expects versions
+  to start with `v` (e.g. `v0.1.0`).
+* Target the main branch.
+* Title the Release the same as the tag: “vX.Y.Z”.
+* Click “set as latest release”.
+* Set the last version as the “Previous tag”.
+* Click “Generate release notes” to autogenerate release notes.
+* Edit the release notes.
+   * Tweak the change description for each if necessary so it summarizes the salient
+     aspect(s) of the change in a single sentence. Detail is not needed as a user could
+     follow the link to the relevant PR. (Potentially take a pass at PR descriptions
+     and revise to increase clarity for users that visit the PRs from the release notes.)
+   * Related commits can be grouped together with a single entry that has links to all
+     relevant PRs (and attributes all relevant contributors).
+   * A summary may be added if warranted.
+   * The items in the list should be broken up into sub-categories. The typical
+     sub-categories to use follow:
+      * **API Improvements**: If the API was changed in any backwards-incompatible ways
+        to improve developer experience, mention those here. Details on breaking changes
+        should accompany the change log with a separate section titled "API Updates".
+        (This category should go away once we reach a stable API and a v1.0.)
+      * **Enhancements**: New features or additions/improvements to existing features.
+      * **Bugfixes**: Self-explanatory -- fixes to defects.
+      * **Other Changes**: Other noteworthy changes in the codebase or tests. In some
+        way these could be considered enhancements, but they are more for maintenance
+        and aid future contributors vs. providing new features to users. Use your
+        best judgement when deciding if something warrants appearing here. Things like
+        dependency updates and the like do _not_ warrant appearing here and should be
+        removed from the auto-generated release notes.
+   * If there were API changes that are not backwards compatible, add an "API Updates"
+     section that enumerates the changes via a bullet list per class/type that was
+     incompatibly changed.
+   * An example of past release notes that contain many of these sections can be
+     reviewed here: https://github.com/connectrpc/connect-kotlin/releases/tag/v0.5.0
+* Click "Save Draft" and then share the link to have the notes reviewed by at least one
+  other [maintainer](https://github.com/connectrpc/connect-kotlin/blob/main/MAINTAINERS.md).
+* After the notes are approved (after some potential iteration and revision), you can
+  finally create the release by clicking "Publish Release".
+
+After the GitHub release has been created, you can verify that the rest of the process
+completes successfully by following the corresponding run of the "release" GitHub
+workflow: https://github.com/connectrpc/connect-kotlin/actions/workflows/release.yml
+
+Once the workflow is complete, the release is complete and artifacts have been
+successfully published to Maven Central.