Describe computed diagram for factorial in preprocessor readme

src/preprocessor/README.md

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+# Fun and learning with the PlantUML preprocessor
+
+The PlantUML preprocessor provides a higher level abstraction to generate the diagram
+descriptions. It can also be used as a programming language to _compute_ the diagrams!
+
+The preprocessor provides variables, conditional expressions, looping constructs, and
+even functions, making it a full fledged Turing complete language.
+
+This also enables having some fun while learning the preprocessor features. In this
+document, I describe some such examples. **All of them are my original work.**
+
+The example diagrams shown here are _computed_ by PlantUML, not manually described in
+puml files.
+
+1. **Compute factorial of a number**: uses and draws the recursion!
+
+   This _hacks_ a state diagram to denote the recursive calls and the computed
+   factorial value for each number. The `!function` feature defines the factorial
+   function that computes the factorial, creates the states and draws the connections
+   between the states.
+
+   https://github.com/dragondive/plantuml_demo/blob/bfd3b1c018fe5893a8f37a9a01a6d9e0074be6eb/src/preprocessor/factorial_demo.puml#L1-L31
+
+   ![State Diagram hack showing factorial computation](diagrams/factorial_demo.svg)