(Suggestion) Add note about creating a .gitignore file to the SharedCode module (step 3)

[gosr]

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I am following the guide (https://play.kotlinlang.org/hands-on/Targeting%20iOS%20and%20Android%20with%20Kotlin%20Multiplatform/03_CreatingSharedCode) currently, and was adding it to source control. Probably it'd be a good idea to add a note in step 3 to create a .gitignore file with the content /build to avoid having that folder being part of the repo.