Add release process steps

CONTRIBUTING.md

@@ -6,4 +6,20 @@ While we're in this phase, we're accepting PRs for clarification in documentatio
 
 ## Project structure
 
-The lumigator project contains several packages: the `lumigator/python/mzai/backend` package that powers the lumigator server functionality, the `lumigator/python/mzai/schemas` package containing the formal schemas for communication with the server through the REST API, and the `lumigator/python/mzai/sdk` package abstracting the REST API for Python applications. Each package holds its own `pyproject.toml` definition.
+The lumigator project contains several packages: the `lumigator/python/mzai/backend` package that powers the lumigator server functionality, the `lumigator/python/mzai/schemas` package containing the formal schemas for communication with the server through the REST API, and the `lumigator/python/mzai/sdk` package abstracting the REST API for Python applications. Each package holds its own `pyproject.toml` definition.
+
+## SDK release steps
+
+When applying a semver tag version starting with 'v' without pre-release or build sections (e.g. `v0.0.0`), the SDK and schemas publishing process to PyPI will start. The CI actions will check that the existing versions in the corresponding `pyproject.toml` files match the git tag used. For example, a git tag of `v0.1.2` requires a line of `version = 0.1.2` within the `pyproject.toml` file under the `project` table.
+
+The process to prepare a release should be:
+
+* Ensure that the right version is selected.
+  * This is kept as a manual step, since versions can be skipped for a number of reasons.
+* Fill in this same version in the `lumigator/python/mzai/schemas/pyproject.toml` and `lumigator/python/mzai/sdk/pyproject.toml` files.
+* Make a local commit.
+* Tag the local commit: `git tag vX.Y.Z`
+* Push the local commit: `git push`
+* Push the local tag: `git push origin tag vX.Y.Z`
+
+The publishing CI action should then trigger. If this action fails, repeat with a new patch version. Tags should not be moved in the repo to avoid inconsistencies with PyPI package versions.