Kotlin: add String article

[joshuadeguzman]

Hi, I have noticed that there is not enough topic included under Kotlin section guide. So here I included a technical article regarding Kotlin's String

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[joshalling]

This LGTM! 👍 👍

[joshalling]

Thanks @joshuadeguzman and congrats on your first contribution to the guide! 👍 👍

[joshuadeguzman]

Thanks @joshalling, looking forward for curating more concise and helpful articles!