docs: Add/update documentation README.md.

docs/README.md

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+# Documentation
+
+This is the documentation **source** for this repository.
+
+The **deployed** version of the documentation for this repository is available at:
+
+* https://www.apollographql.com/docs/
+
+## Documentation for the documentation
+
+This `README.md` is intentionally short since the [documentation for the documentation](https://docs-docs.netlify.com/docs/docs/) provides details for the documentation framework _itself_.  Additional information should generally be added to that documentation rather than here in this `README.md`, in order to provide a centralized resource that benefits all documentation deployments.
+
+## Running locally
+
+For more information, consult the documentation for the documentation, referenced above.
+
+In general though:
+
+* `npm install` in this directory
+* `npm start` in this directory
+* Open a browser to the link provided in the console.
+
+> **Important note:** Changes to the markdown source does not result in an automatic "hot reload" in the browser; it is necessary to reload the page manually in the browser to see it re-rendered.  Additionally, changes to `_config.yml` require stopping the server and restarting with `npm start` again.
+
+## Deploy previews
+
+Documentation repositories should be setup with a "deploy preview" feature which automatically provides "preview" links in the _status checks_ section of pull-requests.
+
+In the event that it's not possible to run the documentation locally, pushing changes to the branch for a pull-request can be a suitable alternative that ensures changes to the documentation are properly rendered.
+