Add readme with installation/development instructions

README.md

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+# g-adopt.github.io
+
+You've reached the repository that powers [the G-ADOPT website][1].
+It's powered by [MkDocs], using the [Material for MkDocs][2]
+theme. API documentation is generated using [mkdocstrings].
+
+## Local development
+
+For simple development on the main content of the site, after cloning
+this repository, create a Python virtual environment, and install the
+required packages into it:
+
+    $ python3 -m venv env
+	$ source env/bin/activate
+	(env) $ python3 -m pip install -r requirements.txt
+
+Start the MkDocs server, and the site will be available at http://localhost:8000
+
+    (env) $ python3 -m mkdocs serve
+
+### API documentation
+
+If you're working on the API documentation, there's an extra step
+regarding the repository from which to build the documentation. By
+default, MkDocs will look for a clone of [g-adopt] at the `g-adopt`
+path inside this repository. If you'd prefer to work with the
+repository at a different path, change the following key in the
+`mkdocs.yml` file:
+
+```yaml
+plugins:
+  - mkdocstrings:
+      handlers:
+        python:
+          paths: ...
+```
+
+## Deployment
+
+Changes to the live website are automatically deployed from pushes to
+the `main` branch.
+
+[1]: https://gadopt.org
+[MkDocs]: https://www.mkdocs.org/
+[2]: https://squidfunk.github.io/mkdocs-material/
+[mkdocstrings]: https://mkdocstrings.github.io/
+[g-adopt]: https://github.com/g-adopt/g-adopt