Add archdoc build instructions

This add instructions on how to build the architecture specification document to the README file.

README.md

@@ -35,6 +35,16 @@ make csim
 
 This will produce an executable in `c_emulator/cheriot_sim` that can be used to run ELF files produced by the CHERIoT compiler.
 
+To build the documentation you must have `biber`, `latexmk` and `pdflatex` and have setup your `opam` environment as above. Then follow the following commands:
+
+```sh
+pushd archdoc
+make
+popd
+```
+
+This will create a file located at `archdoc/cheriot-architecture.pdf`.
+
 ## Contributing
 
 This project welcomes contributions and suggestions.  Most contributions require you to agree to a
@@ -55,4 +65,4 @@ This project may contain trademarks or logos for projects, products, or services
 trademarks or logos is subject to and must follow 
 [Microsoft's Trademark & Brand Guidelines](https://www.microsoft.com/en-us/legal/intellectualproperty/trademarks/usage/general).
 Use of Microsoft trademarks or logos in modified versions of this project must not cause confusion or imply Microsoft sponsorship.
-Any use of third-party trademarks or logos are subject to those third-party's policies.
+Any use of third-party trademarks or logos are subject to those third-party's policies.