Updated the readme with build instructions

Author: MrMatAP

README.md

@@ -18,26 +18,39 @@ This variant of a Python Flask API is code-first and using native Flask
 
 ## How to build this
 
-In bash:
+Building a wheel boils down to this (in bash or Powershell):
 
-```shell
-$ pip install -r build-requirements.txt  # Manually install dependencies (see note in requirements.txt!)
+```bash
+$ pip install -r requirements.txt  # Manually install dependencies (see note in requirements.txt!)
 $ export PYTHONPATH=`pwd`                # In order to find the build-time ci module
 $ export MRMAT_VERSION=1.0.27            # Optional: To influence the version. Normally calculated and set by CI
 $ python -m build -n                     # Use -n in an interactive, virtual environment
 $ ls dist/
 mrmat_python_api_flask-1.0.27.dev0-py3-none-any.whl
 ```
 
-In PowerShell:
-
 ```powershell
 PS> pip install -r build-requirements.txt     # Manually install dependencies (see note in requirements.txt!)
 PS> $Env:PYTHONPATH=(pwd)                     # In order to find the build-time ci module
 PS> $Env:MRMAT_VERSION = "1.0.27"             # Optional: To influence the version. Normally calculated and set by CI
 PS> python -m build
 ```
 
+This project uses the [PEP517 build mechanism](https://www.python.org/dev/peps/pep-0517/), but with a twist so CI is in
+control over the generation of the micro version number. In our case, CI is GitHub Actions, which has the top-level
+environment variables `MAJOR` and `MINOR`. These two are intended to be infrequently updated manually if and when
+it is needed. The micro version is automatically set by the `GITHUB_RUN_NUMBER` which increases for every build
+we do. The `MRMAT_VERSION` environment variable is a concatenation of these three. If the build is via a PR onto
+main then this is the release version. Otherwise `.dev0` is appended to the version.
+
+If you are building locally, then your version will **always** be `0.0.0.dev0` (unless you explicitly set the
+`MRMAT_VERSION` environment variable to something different).
+
+It is necessary for setup.cfg to figure out whether and what `MRMAT_VERSION` is set to. Since this is no longer
+an executable in PEP517, we tell it to use the `ci` module for doing so. The `ci` module simply picks up `MRMAT_VERSION`
+from the environment, no more and the module is explicitly ignored from packaging. However, you must ensure that
+the top-level `ci` module is on the Python path as you build (hence the setup above).
+
 ## How to run this
 
 You have the choice of running this