Write ReadMe about what constitutes the public API and what is an incompatible change

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+# Kotlin Public API binary compatibility validation tool
+
+This tool allows to dump binary API of a Kotlin library that is public in sense of Kotlin visibilities
+and ensure that the binary API wasn't changed in a way that make this change binary incompatible.
+
+## What constitutes the public API
+
+### Classes
+
+A class is considered to be effectively public if all of the following conditions are met:
+
+ - it has public or protected JVM access (`ACC_PUBLIC` or `ACC_PROTECTED`)
+ - it has one of the following visibilities in Kotlin:
+    - no visibility (means no Kotlin declaration corresponds to this compiled class)
+    - *public*
+    - *protected*
+    - *internal*, only in case if the class is annotated with `InlineExposed`
+ - it isn't a local class
+ - it isn't a synthetic class with mappings for `when` tableswitches (`$WhenMappings`)
+ - it contains at least one effectively public member, in case if the class corresponds
+   to a kotlin *file* with top-level members or a *multifile facade*
+ - in case if the class is a member in another class, it is contained in the *effectively public* class
+ - in case if the class is a protected member in another class, it is contained in the *non-final* class
+
+### Members
+
+A member of the class (i.e. a field or a method) is considered to be effectively public
+if all of the following conditions are met:
+
+ - it has public or protected JVM access (`ACC_PUBLIC` or `ACC_PROTECTED`)
+ - it has one of the following visibilities in Kotlin:
+    - no visibility (means no Kotlin declaration corresponds to this class member)
+    - *public*
+    - *protected*
+    - *internal*, only in case if the class is annotated with `InlineExposed`
+
+    > Note that Kotlin visibility of a field exposed by `lateinit` property is the visibility of it's setter.
+ - in case if the member is protected, it is contained in *non-final* class
+ - it isn't a synthetic access method for a private field
+
+## What makes an incompatible change to the public binary API
+
+### Class changes
+
+For a class a binary incompatible change is:
+
+ - changing the full class name (including package and containing classes)
+ - changing the superclass, so that the class no longer has the previous superclass in
+   the inheritance chain
+ - changing the set of implemented interfaces so that the class
+   no longer implements interfaces it had implemented before
+ - changing one of the following access flags:
+    - `ACC_PUBLIC`, `ACC_PROTECTED`, `ACC_PRIVATE` — lessening the class visibility
+    - `ACC_FINAL` — making non-final class final
+    - `ACC_ABSTRACT` — making non-abstract class abstract
+    - `ACC_INTERFACE` — changing class to interface and vice versa
+    - `ACC_ANNOTATION` — changing annotation to interface and vice versa
+
+### Class member changes
+
+For a class member a binary incompatible change is:
+
+ - changing its name
+ - changing its descriptor (erased return type and parameter types for methods);
+   this includes changing field to method and vice versa
+ - changing one of the following access flags:
+    - `ACC_PUBLIC`, `ACC_PROTECTED`, `ACC_PRIVATE` — lessening the member visibility
+    - `ACC_FINAL` — making non-final field or method final
+    - `ACC_ABSTRACT` — making non-abstract method abstract
+    - `ACC_STATIC` — changing instance member to static and vice versa