Updated Readme as .fvm directory was not created

mr-goorequested to merge
mr-goo/kickandflutter:update_readme into main

Updated the README file as, following the instructions, fvm flutter pub get would fail to create the .fvm directory.

Added the ```fvm use`` command as that properly created the directory

It's unknown why the pubspec.yaml was only used to download the SDK, but not create the directory at the project root.

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