document the readthedocs integration for the guidebook

Add a section to the README describing how the guidebook is hosted and
built.

Closes #3

README.md

@@ -19,3 +19,29 @@ $ mkdocs serve
 ```
 
 The documentation is now available at http://localhost:8000/.
+
+## Hosting
+
+The guidebook is hosted as [a
+project](https://readthedocs.org/projects/uis-devops-division-guidebook/) on
+readthedocs.org (RTD) and is automatically built on each commit to master. RTD
+natively supports mkdocs documentation and so, beyond informing RTD that this is
+a mkdocs project, no configuration is required to build the docs.
+
+The ``guidebook.devops.uis.cam.ac.uk`` DNS record is a CNAME pointing to
+``readthedocs.io`` following the [custom domain
+documentation](https://docs.readthedocs.io/en/stable/custom_domains.html) from
+RTD.
+
+Currently, the RTD configuration is manual. To gain access to the project, create
+a RTD account and ask an existing administrator to add you as a maintainer to
+the project.
+
+RTD knows when to build the documentation by means of a
+[webhook](https://docs.readthedocs.io/en/stable/webhooks.html). This has to be
+manually added to the Developer Hub project as an
+[integration](https://gitlab.developers.cam.ac.uk/uis/devops/docs/guidebook/hooks/10/edit).
+The Developer Hub project should be configured to POST to the RTD webhook on
+push events. [GitLab-specific
+documentation](https://docs.readthedocs.io/en/stable/webhooks.html#gitlab) is
+provided by RTD.