Add a release process guide (#243)

docs/contribute.md

@@ -14,6 +14,7 @@ The guidelines document is organized as the following sections:
 - [Pull Requests](#pull-requests)
 - [Testing](#testing)
 - [Versioning](#versioning)
+- [PyPI Release Process](#pypi-release-process)
 - [Public APIs](#public-apis)
 
 
@@ -128,6 +129,24 @@ We don't have any fancy testing framework set up yet. If you want to run tests l
 
 We use semantic versioning, as described in [versioning.md](https://github.com/onnx/turnkeyml/blob/main/docs/versioning.md).
 
+## PyPI Release Process
+
+TurnkeyML is provided as a package on PyPI, the Python Package Index, as [turnkeyml](https://pypi.org/project/turnkeyml/). The release process for pushing an updated package to PyPI is mostly automated, however (by design), there are a few manual steps.
+1. Make sure the version number in [version.py](https://github.com/onnx/turnkeyml/blob/main/src/turnkeyml/version.py) has a higher value than the current [PyPI package](https://pypi.org/project/turnkeyml/).
+    - Note: if you don't take care of this, PyPI will reject the updated package and you will need to start over from Step 1 of this guide.
+    - If you are unsure what to set the version number to, consult [versioning.md](https://github.com/onnx/turnkeyml/blob/main/docs/versioning.md).
+1. Make sure all of the changes you want to release have been merged to `main`.
+1. Go to the [TurnkeyML GitHub front page](https://github.com/onnx/turnkeyml) and click "Releases" in the side bar.
+1. At the top of the page, click "Draft a new release".
+1. Click "Choose a tag" (near the top of the page) and write `v` (lowercase), followed by the contents of the string in [version.py](https://github.com/onnx/turnkeyml/blob/main/src/turnkeyml/version.py).
+  - For example, if `version.py` contains `__version__ = "4.0.5"`, the string is `4.0.5` and you should write `v4.0.5` into the text box.
+1. Click the "+Create new tag:... on publish" button that appears under the next box.
+1. Click "Generate release notes" (near the top of the page). Modify as necessary. Make sure to give credit where credit is due!
+1. Click "Publish release" (green button near the bottom of the page). This will start the release automations, in the form of a [Publish Distributions to PyPI Action](https://github.com/onnx/turnkeyml/actions/workflows/publish-to-test-pypi.yml).
+  - Note: if you forgot the "v" in the "Choose a tag" step, this Action will not run.
+1. Wait for the Action launched by the prior step to complete. Go to [the turnkeyml PyPI page](https://pypi.org/project/turnkeyml/) and spam refresh. You should see the version number update.
+  - Note: `pip install turnkeyml` may not find the new update for a few more minutes.
+
 ## Public APIs
 
 The following public APIs are available for developers. The maintainers aspire to change these as infrequently as possible, and doing so will require an update to the package's major version number.