Add gradle plugin portal badge

Author: Abduqodiri Qurbonzoda

README.md

@@ -2,6 +2,7 @@
 [![GitHub license](https://img.shields.io/badge/license-Apache%20License%202.0-blue.svg?style=flat)](https://www.apache.org/licenses/LICENSE-2.0)
 [![Build status](https://teamcity.jetbrains.com/guestAuth/app/rest/builds/buildType:(id:KotlinTools_KotlinxCollectionsImmutable_Build_All)/statusIcon.svg)](https://teamcity.jetbrains.com/viewType.html?buildTypeId=KotlinTools_KotlinxCollectionsImmutable_Build_All)
 [![Maven Central](https://img.shields.io/maven-central/v/org.jetbrains.kotlinx/kotlinx-benchmark-runtime.svg?label=Maven%20Central)](https://search.maven.org/search?q=g:%22org.jetbrains.kotlinx%22%20AND%20a:%22kotlinx-benchmark-runtime%22)
+[![Gradle Plugin Portal](https://img.shields.io/maven-metadata/v?label=Gradle%20Plugin&metadataUrl=https://plugins.gradle.org/m2/org/jetbrains/kotlinx/kotlinx-benchmark-plugin/maven-metadata.xml)](https://plugins.gradle.org/plugin/org.jetbrains.kotlinx.benchmark)
 
 
 > **_NOTE:_**   The latest version of the library changes how the plugin and runtime library dependencies are declared: