Merge #552: Readme: mention theme used and link to its documentation

9865f85 Readme: mention theme used and link to its documentation (David A. Harding)

Pull request description:

  Closes #525 ; CC: @karel-3d

  Links to the website and documentation for the upstream of the theme we use.  We haven't upgraded the files related to the theme since the website went live in Jan 2016, so I also mention that the current documentation might not completely match the theme files/settings.

Tree-SHA512: e2ee08e86cc64df640cb526c4e5f262b96c6d1c0e3415109a0c4088e128470971e6253bf2a84821a500657c3d409d0bd8f4b94d7f33fe070f195cb24323eadfe

Author: harding

README.md

@@ -87,3 +87,18 @@ pages are built and checked.
 ## Contributing
 
 Contributions welcome. Please see [CONTRIBUTING.md](/CONTRIBUTING.md) for details.
+
+## References
+
+The website uses an old version of the [Minimal Mistakes theme][].  The
+theme's website provides [documentation][mm docs], including information
+about [configuration variables][mm config], creating [pages and posts][mm
+content], adding [new Javascript][mm js], and more.  Note that
+current documentation may describe features not available in the old
+version of the theme used by the website.
+
+[minimal mistakes theme]: https://mmistakes.github.io/minimal-mistakes/
+[mm docs]: https://mmistakes.github.io/minimal-mistakes/docs/quick-start-guide/ 
+[mm config]: https://mmistakes.github.io/minimal-mistakes/docs/configuration/
+[mm content]: https://mmistakes.github.io/minimal-mistakes/docs/posts/
+[mm js]: https://mmistakes.github.io/minimal-mistakes/docs/javascript/