added a readme to the calculator example

Author: rydb

examples/diagram/calculator/README.md

@@ -0,0 +1,11 @@
+# Calculator example
+
+An example showing spawning a `bevy_impulse` service for managing a calculator.
+
+to run the example, open a terminal focused on this folder and run:
+
+```
+cargo run -- multiply3.json n
+```
+
+where `n` is a number you wish to multiply by.