Added minimum required Gradle version (#182)

Author: fzhinkin

README.md

@@ -9,6 +9,7 @@ The tool allows dumping binary API of a JVM part of a Kotlin library that is pub
 
 ## Contents
 
+* [Requirements](#requirements)
 * [Setup](#setup)
   * [Tasks](#tasks)
   * [Optional parameters](#optional-parameters)
@@ -21,6 +22,10 @@ The tool allows dumping binary API of a JVM part of a Kotlin library that is pub
   * [Class member changes](#class-member-changes)
 * [Building locally](#building-the-project-locally)
 
+## Requirements
+
+Binary compatibility validator plugin requires Gradle `6.0` or newer. 
+
 ## Setup
 
 Binary compatibility validator is a Gradle plugin that can be added to your build in the following way: