Update INTEROP.md
Update INTEROP.md

Describe working with `CValue`.

JetBrains/kotlin-native,jiaminglu/kotlin-native,jiaminglu/kotlin-native,JuliusKunze/kotlin-native,JuliusKunze/kotlin-native,JetBrains/kotlin-native,JetBrains/kotlin-native,jiaminglu/kotlin-native,wiltonlazary/kotlin-native,JuliusKunze/kotlin-native,jiaminglu/kotlin-native,JetBrains/kotlin-native,JuliusKunze/kotlin-native,JetBrains/kotlin-native,jiaminglu/kotlin-native,JetBrains/kotlin-native,JuliusKunze/kotlin-native,wiltonlazary/kotlin-native,JuliusKunze/kotlin-native,JuliusKunze/kotlin-native,jiaminglu/kotlin-native,wiltonlazary/kotlin-native,JuliusKunze/kotlin-native,jiaminglu/kotlin-native,jiaminglu/kotlin-native,wiltonlazary/kotlin-native,JetBrains/kotlin-native,JetBrains/kotlin-native,wiltonlazary/kotlin-native,JetBrains/kotlin-native