Docs: explain: pagy + retype, add contribution instructions

Why? To document hidden steps.

README.md

@@ -358,6 +358,26 @@ See also the [How To Page](https://ddnexus.github.io/pagy/docs/how-to)
 - For simple contribution you can quickly check your changes with the [Pagy::Console](https://ddnexus.github.io/pagy/docs/api/console/) or with the single file [pagy_standalone_app.ru](https://github.com/ddnexus/pagy/blob/master/apps/pagy_standalone_app.ru).
 - If you Create A Pull Request, please ensure that the "All checks have passed" indicator gets green light on the Pull Request page (if it's not enabled, a maintainer will enable it for you).
 
+
+### Documentation Contributions
+
+Documentation contributions or suggestions are welcome.
+
+1. Download [`retype`](https://retype.com/guides/cli/)
+2. `retype start` in the pagy root directory.
+
+And your docs should appear in a browser.
+
+#### Primer on how Pagy's Documentation works
+
+Pagy's documentation is built on [retype](https://retype.com/).
+
+Pagy uses a [github action](https://github.com/ddnexus/pagy/blob/master/.github/workflows/retype-action.yml) to trigger the build process i.e. the docs need to be "built". Why? Because the docs are written in [markdown format](https://en.wikipedia.org/wiki/Markdown) - however, markdown is not sufficient to deliver html to your browser. Retype does the hard work of converting that markdown into html with styling.
+
+Once built, then [Github pages](https://pages.github.com/) delivers it to you. But we must tell Github to deliver the html that has been created. Where is the html located? The html - i.e. the "built" site - is located in the root directory of the [`docs-site` branch](https://github.com/ddnexus/pagy/tree/docs-site) of the Pagy repository. The admin of the repository can set the branch that is used in the settings section of the repository.
+
+More details on the docs are available on the [retype website](https://retype.com/guides/github-actions/). The Pagy repository is using the Open Source version, subject to certain limitations (i.e. total page count). [Pro version](https://retype.com/pro/) opens up other benefits.
+
 </details>
 
 <details>