Update README.md

Author: iamshaunjp

README.md

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 # flutter-beginners-tutorial
 All course files for the Flutter Beginners playlist on The Net Ninja YouTube channel.
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+## How to use these files
+Each lesson in the playlist has it's own branch in this repository. To see the code for that lesson, choose the appropriate branch. E.g. to see the code for lesson 15, checkout the lesson-15 branch.
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+**Link to playlist on YouTube - **