Update the README.md with links to the generated documents.

We now have some documents that are generated in GitHub Pages, and the
README.md will now link to it.

README.md

@@ -1,11 +1,41 @@
 # RISC OS supplementary documentation
 
+[![Supplementary documents](https://img.shields.io/badge/Generated-Documents-yellow)](https://gerph.github.io/riscos-prminxml-staging/)
+
 ## Introduction
 
 This repository hold supplementary documentation in PRM-in-XML format.
 It is a staging area for conversions of documentation.
+The documents are meant to be generated by the PRM-in-XML tool, which
+converts the content to HTML files which can be read more easily.
+
+The [Releases](https://github.com/gerph/riscos-prminxml-staging/releases)
+on GitHub include archives of the generated documentation, in addition to the pages
+on the [generated website](https://gerph.github.io/riscos-prminxml-staging/).
+
+## Building the examples
+
+The example documents can be generated on a Linux system, although this may
+need to install packages.
+
+    ./build.sh
+
+This will generate the documents in the directory `output`.
+
+If you do not wish to install packages, you may manually build with the
+following alternatives.
+
+### Building with the PRM-in-XML tool
+
+To directly generate the documentation, the
+[PRM-in-XML tool](https://github.com/gerph/riscos-prminxml-tool)
+is required to process these files.
+
+To build them, use a command like:
+
+    riscos-prminxml src/acorn/cnp/cut_n_paste.xml
 
-## Building the documentation
+### Building with docker
 
 Building the documentation can be performed on most modern POSIX systems with Docker.