feat: add the contributing page to the repo.

Author: IainTheTechWriter

CONTRIBUTING.md

@@ -0,0 +1,55 @@
+# Contributing 
+
+The release process for Bitvavo SDK for Python is semi-automatic. You manually start the GitHub Action that creates 
+the SDK package and uploads it to Python package managers.
+
+## Prerequisites
+
+To release the SDK you need to:
+
+- Complete the instructions in the [README Prerequisites](./README.md#prerequisites)
+
+## Release a new version of the SDK
+
+To update the SDK and publish your changes to the Bitvavo developer community:
+
+1. **Make your updates to the SDK**
+
+    Create a branch and implement your changes to the SDK. When you have tested your code updates, make a pull request 
+    to the `master` branch.
+
+1. **Create a test package**
+
+   1. When your [pull request](https://github.com/bitvavo/python-bitvavo-api/pulls) is approved, merge your changes to 
+      the master branch.
+   2. Note the release number of the 
+      [latest version of the SDK published to pypi](https://pypi.org/project/python-bitvavo-api/#history)
+      and using [semver](https://semver.org/) as your guide, decide on a version number for this release.
+   4. In Actions, [run the release action](https://github.com/bitvavo/python-bitvavo-api/actions/workflows/release.yml) 
+      using your new release number.
+     The release action builds the SDK package and uploads to https://test.pypi.org/project/python-bitvavo-api/.
+   
+1. **Test the package**
+
+   1. In a [clean virtual environment](https://virtualenv.pypa.io/en/stable/user_guide.html), create a new directory, 
+      then download the test package:
+
+       ```python3 -m pip install --index-url https://test.pypi.org/simple/ python-bitvavo-api```
+
+   1. Follow the instructions in [Create a simple Bitvavo implementation](./README.md#get-started) and make a sample 
+      app.
+       When the functionality you implemented works, the package is good. If you cannot make a 
+      call, the debugger is your friend.
+   
+1. **Publish the SDK**
+
+   1. Open the draft release created when you ran the release action.
+   1. Add a brief description about the release.
+   
+      This description is visible to the world in https://github.com/bitvavo/python-bitvavo-api/releases.
+   1. Click **Publish**.
+
+      This starts the Publish action. You see the 
+      [new version available on pypi](https://pypi.org/project/python-bitvavo-api/).
+
+Now send the release message in Slack, yay.