examples/calculator: intro a README.md file

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>

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+Portable calculator built using _app.nit_
+
+# Project structure
+
+* `calculator_logic` defines `CalculatorContext` with all the business logic of a calculator.
+  It takes as input operations and numbers, and outputs the text to display.
+* `calculator` implements the portable graphical interface using the _app.nit_ framework
+* `calculator_test` test `CalculatorContext` as a black box.
+
+# Usage
+
+1. Compile with a call to `make`.
+2. Run with either:
+       1. The desktop version, `bin/calculator`.
+       2. The Android version by install the APK at `bin/calculator.apk`.