Update README.md

Author: stephengruppetta

python-mappings/README.md

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-Code defining the `PizzaMenu` class in the tutorial Python Mappings: A Comprehensive Guide
+# Python Mappings: A Comprehensive Guide
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+This folder contains source code for the Real Python tutorial [Python Mappings: A Comprehensive Guide](https://realpython.com/python-mappings-comprehensive-guide/).
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+For more code, intermediate steps, and additional explanation, check out the associated tutorial.