docs: document alpha release process (ratatui-org#542)

Fixes ratatui-org#412

RELEASE.md

@@ -26,3 +26,20 @@ actions](.github/workflows/cd.yml) and triggered by pushing a tag.
 1. Push the tag: `git push --tags`
 1. Wait for [Continuous Deployment](https://github.com/ratatui-org/ratatui/actions) workflow to
    finish.
+
+## Alpha Releases
+
+Alpha releases are automatically released every Saturday via [cd.yml](./.github/workflows/cd.yml)
+and can be manually be created when necessary by triggering the [Continuous
+Deployment](https://github.com/ratatui-org/ratatui/actions/workflows/cd.yml) workflow.
+
+We automatically release an alpha release with a patch level bump + alpha.num weekly (and when we
+need to manually). E.g. the last release was 0.22.0, and the most recent alpha release is
+0.22.1-alpha.1.
+
+These releases will have whatever happened to be in main at the time of release, so they're useful
+for apps that need to get releases from crates.io, but may contain more bugs and be generally less
+tested than normal releases.
+
+See [#147](https://github.com/ratatui-org/ratatui/issues/147) and
+[#359](https://github.com/ratatui-org/ratatui/pull/359) for more info on the alpha release process.