Add conversion notes to Readme

13_Bounce/csharp/README.md

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+# Bounce
+
 Original source downloaded [from Vintage Basic](http://www.vintage-basic.net/games.html)
 
 Conversion to [Microsoft C#](https://docs.microsoft.com/en-us/dotnet/csharp/)
+
+## Conversion notes
+
+### Mode of Operation
+
+This conversion performs the same function as the original, and provides the same experience, but does it in a different
+way.
+
+The original BASIC code builds the graph as it writes to the screen, scanning each line for points that need to be
+plotted.
+
+This conversion steps through time, calculating the position of the ball at each instant, building the graph in memory.
+It then writes the graph to the output in one go.
+
+### Failure Modes
+
+The original BASIC code performs no validation of the input parameters. Some combinations of parameters produce no
+output, others crash the program.
+
+In the spirit of the original this conversion also performs no validation of the parameters, but it does not attempt to
+replicate the original's failure modes. It fails quite happily in its own way.