Update docs/README.md with information on the site and how to build it

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 # Synapse Documentation
 
-This directory contains documentation specific to the `synapse` homeserver.
+**The documentation is currently hosted [here](https://matrix-org.github.com/synapse).**
+Please update any links to point to the new website instead.
 
-All matrix-generic documentation now lives in its own project, located at [matrix-org/matrix-doc](https://github.com/matrix-org/matrix-doc)
+## About
 
-(Note:  some items here may be moved to [matrix-org/matrix-doc](https://github.com/matrix-org/matrix-doc) at some point in the future.)
+This directory currently holds a series of markdown files documenting how to install, use
+and develop Synapse, the reference Matrix homeserver. The documentation is readable directly
+from this repository, but it is recommended to instead browse through the
+[website](https://matrix-org.github.com/synapse) for easier discoverability.
+
+## Adding to the documentation
+
+Most of the documentation currently exists as top-level files, as when organising them into
+a structured website, these files were kept in place so that existing links would not break.
+The rest of the documentation is stored in folders, such as `setup`, `usage`, and `development`
+etc. **All new documentation files should be placed in structured folders.** For example:
+
+To create a new user-facing documentation page about a new Single Sign-On protocol named
+"MyCoolProtocol", one should create a new file with a relevant name, such as "my_cool_protocol.md".
+This file might fit into the documentation structure at:
+
+- Usage
+  - Configuration
+    - User Authentication
+      - Single Sign-On
+        - **My Cool Protocol**
+
+Given that, one would place the new file under
+`usage/configuration/user_authentication/single_sign_on/my_cool_protocol.md`.
+
+Note that the structure of the documentation (and thus the left sidebar on the website) is determined
+by the list in [SUMMARY.md](SUMMARY.md). The final thing to do when adding a new page is to add a new
+line linking to the new documentation file:
+
+```markdown
+- [My Cool Protocol](usage/configuration/user_authentication/single_sign_on/my_cool_protocol.md)
+```
+
+## Building the documentation
+
+The documentation is built with [mdbook](https://rust-lang.github.io/mdBook/), and the outline of the
+documentation is determined by the structure of [SUMMARY.md](SUMMARY.md).
+
+First, [get mdbook](https://github.com/rust-lang/mdBook#installation). Then, **from the root of the repository**,
+build the documentation with:
+
+```sh
+mdbook build
+```
+
+The rendered contents will be outputted to a new `book/` directory at the root of the repository. You can
+browse the book by opening `book/index.html` in a web browser.
+
+You can also have mdbook host the docs on a local webserver with hot-reload functionality via:
+
+```sh
+mdbook serve
+```
+
+The URL at which the docs can be viewed at will be logged.
+
+## Configuration and theming
+
+The look and behaviour of the website is configured by the [book.toml](../book.toml) file
+at the root of the repository. See
+[mdbook's documentation on configuration](https://rust-lang.github.io/mdBook/format/config.html)
+for available options.
+
+The site can be themed and additionally extended with extra UI and features. See
+[website_files/README.md](website_files/README.md) for details.