add docs build instructions to docs/README

docs/README.md

@@ -1 +1,39 @@
-These markdown files (*.md) contain relative hyperlinks. Any relative hyperlinks will not work when viewing these markdown files in github.
+# Please browse the docs from either http://nrf24.github.io/RF24 or https://rf24.rtfd.io
+
+## Intended for use in Doxygen
+
+The markdown files (\*.md) in this docs folder contain relative hyperlinks. Any relative hyperlinks will not work when viewing these markdown files in github.
+
+----
+
+## Building the docs
+
+Navigate to the repo's docs folder:
+
+```shell
+cd docs
+```
+
+Install Doxygen (v1.9.5 or newer) and run the following command from the repo's docs folder:
+
+```shell
+doxygen
+```
+
+The Doxygen HTML output is now in docs/html. The Doxygen XML output in docs/sphinx/xml can be optionally used to generate Sphinx output.
+
+### Generating Sphinx docs (optional)
+
+To build the Sphinx docs based on Doxygen's XML output, first install the necessary python dependencies.
+
+``` shell
+python -m pip install docs/sphinx/requirements.txt
+```
+
+Now build the Sphinx docs (after building the Doxygen output) from the repo's docs folder:
+
+```shell
+sphinx-build sphinx _build/html
+```
+
+The Sphinx HTML output now exists in docs/_build/html.