doc(README): add UML class diagram image

Author: rouson

README.md

@@ -42,13 +42,20 @@ into submodules.
 As compared to the original code, this repository also adds
 1. A [Fortran Package Manager] build system,
 2. Tests based on the [Vegetables] unit-testing software,
-3. Documentation generated by [FORD] and deployed to the web via GitHub Actions, and
+3. Documentation generated by [`ford`] and deployed to the web via GitHub Actions, and
 4. Quality control via continuous integration testing using GitHub Actions.
 
 Documentation
 -------------
 See [Reference Counter's GitHub Pages site] for HTML documentation generated with [`ford`].
 
+See the [doc/] subdirectory for a [PlantUML] script that generates the Unified Modeling Langauge (UML) class diagram below of the three derived types in reference-counter.  
+
+![class_diagram](https://user-images.githubusercontent.com/13108868/165135689-4d2e85fe-6946-472f-a154-aaabebf6d4f5.png)
+
+The above image was created with the PlantuML package in the [Atom] editor.
+
+
 Compiler Status
 ---------------
 Correct execution of the Reference Counter library code requires comprehensive
@@ -110,5 +117,8 @@ fpm test --compiler flang --flag -cpp
 [3]: https://doi.org/10.1109/MCSE.2012.33
 [Fortran Package Manager]: https://github.com/fortran-lang/fpm
 [Vegetables]: https://gitlab.com/everythingfunctional/vegetables
-[FORD]: https://github.com/Fortran-FOSS-Programmers/ford
+[`ford`]: https://github.com/Fortran-FOSS-Programmers/ford
 [Reference Counter's GitHub Pages site]: https://sourceryinstitute.github.io/reference-counter
+[Atom]: https://atom.io
+[PlantUML]: https://plantuml.com
+[doc/]: ./doc