doc: Add initial RELEASE.md

RELEASE.md

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+# Releases
+
+## How to cut an individual release
+
+### Versioning strategy
+
+Releases of `cortex-jsonnet` are versioned to match the compatible version of Cortex. The release of `cortex-jsonnet`tagged `1.6.0` should match compatible with the release of Cortex tagged  `1.6.0`.
+A release of `cortex-jsonnet` should follow shortly after a release of Cortex.
+
+### Publish a release
+
+1. Create a branch for the new release.
+2. Update the `CHANGELOG.md`.
+   - Add a new section for the new release so that "## master / unreleased" is blank and at the top.
+   - The new release section should say "## x.y.0 / YYYY-MM-DD".
+   - Ensure changelog entries for the new release are in this order:
+     * `[CHANGE]`
+     * `[FEATURE]`
+     * `[ENHANCEMENT]`
+     * `[BUGFIX]`
+3. Update `cortex/images.libsonnet` to the released Cortex version.
+4. Open a Pull Request for the your branch.
+5. Once your Pull Request has been merged, checkout the merge commit and tag a release. Refer to [How to tag a release](#how-to-tag-a-release).
+6. Build the `cortex-mixin.zip` for the release.
+
+    ```console
+   $ make build-mixin
+   ```
+7. Add the `cortex-mixin.zip` and release change log to the GitHub release.
+   - Edit the release in GitHub by going to https://github.com/grafana/cortex-jsonnet/releases/edit/x.y.z
+
+### How to tag a release
+
+> **Note:** Unlike Cortex, release tags are not prefixed with a `v`.
+
+You can do the tagging on the commandline:
+
+```console
+$ tag="x.y.z"
+$ git tag -s v"${tag}" -m v"${tag}"
+$ git push origin v"${tag}"
+```