include multiplatform specific information

[RivuChk]

I felt set up information was missing for multiplatform projects, I obtained these by viewing multiple samples and from kotlin-slack, so decided to add them. Please do let me know if I missed something or if something is not correct.

[RivuChk]

Hey @evant Did you have a chance to review it? Is there anything that I should change/add in terms of Multiplatform documentation? Please do let me know, sorry if I bothered you.

[evant]

Took the liberty of making some changes and merged, feel free to suggest changes through another issue or pr if you feel like there's something I missed!