Update README.md

Author: PhilReedData

README.md

@@ -10,8 +10,17 @@ Built using [Jupyter Book 1](https://jupyterbook.org/). For additional guidance,
 - [Sphinx Design (the engine behind Jupyter Book)](https://sphinx-design.readthedocs.io/en/latest/) including [CSS classes](https://sphinx-design.readthedocs.io/en/pydata-theme/css_classes.html)
 - [MyST Syntax Extensions](https://myst-parser.readthedocs.io/en/latest/syntax/optional.html) even though we are using Markdown, not MyST
 
+## Running locally and on GitHub Pages
+
 To run locally, install Jupyter Book then build:
 ```shell
 pip install -U jupyter-book
 jupyter-book build TeSS_docs/
 ```
+
+We are hosting on GitHub Pages (using [Actions](https://github.com/ElixirTeSS/docs/blob/main/.github/workflows/main.yml)), using a different build command that means the `.html` part of the paths are not included. For example, we have page URLs like https://elixirtess.github.io/docs/accounts/user/ instead of ~~https://elixirtess.github.io/docs/accounts/user.html~~ -- this is achieved by rendering each page as a directory with its own `index.html` page. When running locally, this doesn't work so well, so use the default build command shown above.
+
+The build command in the GitHub Action includes:
+```shell
+jupyter-book build --builder dirhtml TeSS_docs/
+```