Document release process

We should document our current release process:

release-plz generates a release PR, with a changelog and handles bumping the version number on all rust projects in the loam repo whenever a PR is merged
this PR needs to be merged to cut the new release
the npm loam-cli package version still needs to be updated manually
once that happens we can run the GH action to build and publish the npm package: https://github.com/loambuild/loam/actions/workflows/npm.yml

loambuild / loam