Update README.md and document the release process in more detail (#13)

See discussion at python/mypy#20069 for additional context.

Author: JukkaL

README.md

@@ -1,14 +1,38 @@
 # librt: mypyc runtime library
 
-This library contains efficient C implementations of various Python standard
-library classes and functions. Mypyc can use these fast implementations when
-compiling Python code to native extension modules.
-
-This repository is used to build/publish mypyc runtime library. Development happens
-in [mypy repository](https://github.com/python/mypy). Code is then perodically
-synced from `mypyc/lib-rt` [subdirectory there](https://github.com/python/mypy/tree/master/mypyc/lib-rt).
-Issues should be reported to mypyc [issue tracker](https://github.com/mypyc/mypyc/issues).
-
-Note: wheels are built on each PR and push to master. To publish to PyPI:
-* Create a tag matching the version in `pyproject.toml`.
-* Trigger the `buildwheels.yml` workflow manually.
+This library contains basic functionality that is useful in code compiled
+using mypyc, and efficient C implementations of various Python standard library
+classes and functions. Mypyc can produce faster extensions when you use `librt` in
+the code you compile. `librt` also contains some internal library features used by mypy.
+
+This repository is only used to build and publish the mypyc runtime library. Development
+happens in the [mypy repository](https://github.com/python/mypy). Code is then perodically
+synced from the `mypyc/lib-rt`
+[subdirectory in the mypy repository](https://github.com/python/mypy/tree/master/mypyc/lib-rt).
+
+Report any issues in the [mypyc issue tracker](https://github.com/mypyc/mypyc/issues).
+
+## Making a release
+
+1. As a prerequisite, there generally will be some changes in the mypy repository under `mypyc/lib-rt`
+  that you want to release.
+2. Run the `sync-mypy.py` script in this repository to sync changes from the mypy repository.
+3. Bump the version number in `pyproject.toml` in this repository.
+4. Update `smoke_tests.py` (optional but recommended for new features). Here's how to run tests:
+    * Activate a dedicated virtualenv (don't reuse your mypy virtualenv).
+    * `pip install -U ./lib-rt`
+    * `pip install pytest mypy-extensions`
+    * `pytest smoke_tests.py`
+5. Commit and push (pushing directly to master is fine).
+6. Wait until all [builds](https://github.com/mypyc/librt/actions) complete successfully
+   (no release is triggered yet).
+7. Once builds are complete, tag the release (`git tag vX.Y.Z`; `git push origin vX.Y.Z`).
+8. Go to the ["Actions" tab](https://github.com/mypyc/librt/actions) and click "Build wheels"
+   on the left.
+9. Click "Run workflow" and pick the newly created tag from the drop-down list. This will build
+   *and upload* the wheels.
+10. After the workflow completes, verify that `pip install -U librt` installs the new version from PyPI.
+11. Create a PR to update the `librt` version in `mypy-requirements.txt`, `test-requirements.txt` and
+  `pyproject.toml` (`dependencies`, and `requires` under `build-system`) in the mypy repository.
+
+The process should take about 20 minutes.