Add contribute section, include new page on documentation system

docs/README.md

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+# Sming Documentation
+
+This directory contains the build system for Sming's documentation.
+It is intended to be read online via (Read The Docs)[https://sming.readthedocs.io].
+
+The `source` directory contains some top-level files however most of the
+information is obtained from the various README files associated with the source code.
+
+## Setup
+
+### make
+
+Should already be available as part of Sming build system
+
+### doxygen
+
+See http://www.doxygen.nl/
+
+For Linux:
+
+	sudo apt install doxygen
+
+### Python 3 + Sphinx
+
+For Linux, run `sudo apt install python3-sphinx python3-pip python3-setuptools`.
+
+For Windows, see [Sphinx Installation Guide](https://www.sphinx-doc.org/en/master/usage/installation.html).
+
+### Requirements
+
+Install other required packages:
+
+	pip3 install -r requirements.txt
+
+
+## Building
+
+Build the documentation like this:
+
+	make html
+
+This will:
+
+* Pull in and patch every submodule
+* Generate doxygen API information (in XML format)
+* Build sphinx documents in HTML format
+
+If you make changes to any source documentation files these will be
+picked up automatically. However, if you make any changes to source
+file documentation then you'll need to re-build the doxygen information
+like this:
+
+	make api -B
+	make html