Update forward compatibility notes (#2964)

Fixes #2924

Author: sandwwraith

README.md

@@ -77,7 +77,6 @@ You can find auto-generated documentation website on [kotlinlang.org](https://ko
 
 [New versions](https://plugins.gradle.org/plugin/org.jetbrains.kotlin.plugin.serialization) of the serialization plugin are released in tandem with each new Kotlin compiler version.
 
-Using Kotlin Serialization requires Kotlin compiler `1.4.0` or higher.
 Make sure you have the corresponding Kotlin plugin installed in the IDE, no additional plugins for IDE are required.
 
 ### Gradle

docs/compatibility.md

@@ -88,5 +88,10 @@ This would be indicated by a compiler error specific to a particular feature.
 * New core library versions may or may not require Kotlin compiler plugin update,
 depending on a particular release.
 We'll try to avoid these situations; however, in case of some unexpected issues, it may be necessary.
-So it is possible to have a situation where upgrading serialization runtime from `1.x` to `1.y` requires an update of Kotlin version from `1.4.0` to `1.4.x`.
+So it is possible to have a situation where upgrading serialization runtime from `1.a.0` to `1.b.0` requires an update of Kotlin version from `1.x.0` to `1.y.0` (`y` > `x`).
 The compiler can detect such problems and will inform you if its version is incompatible with a current version of core library.
+
+Note that according to [general rules for Kotlin binaries](https://kotlinlang.org/docs/kotlin-evolution-principles.html#compatibility-options),
+library is mostly forwards compatible with the next language release, but not later ones. 
+It means that if kotlinx.serialization `1.x` was compiled with Kotlin `2.Y`, it is compatible with Kotlin `2.(Y + 1)`, but not
+`2.(Y + 2)`.