Add README section explaining differences from Java Atomics

[jaccozilla]

It would be helpful to have a README section comparing atomicfu vs Java Atomics. The current readme states:

Code it like AtomicReference/Int/Long, but run it in production efficiently as AtomicXxxFieldUpdater on Kotlin/JVM and as plain unboxed values on Kotlin/JS.

It would be helpful to have more information on why AtomicXxxFieldUpdater is more efficient then Java Atomics on Kotlin/JVM. Also any other differences to consider when choosing to use atomicfu over Java Atomics when building a Kotlin/JVM project

[elizarov]

This library is targeted to developers who already know what AtmoicXxxFieldUpdater is and why it should be used. It is not a general-purpose library for wide audience. However, having said that, there should be more explanation done, indeed, if we were to retarget this library to a larger audience and make it an "official" JetBrains library for Kotlin.