add gradle and maven badges.

README.md

@@ -1,6 +1,8 @@
 # Fray: General-Purpose Concurrency Testing 
 
 [![Build](https://github.com/cmu-pasta/fray/actions/workflows/main.yml/badge.svg)](https://github.com/cmu-pasta/fray/actions/workflows/main.yml)
+[![Gradle](https://img.shields.io/gradle-plugin-portal/v/org.pastalab.fray.gradle)](https://plugins.gradle.org/plugin/org.pastalab.fray.gradle)
+[![Maven](https://img.shields.io/maven-central/v/org.pastalab.fray.maven/fray-plugins-maven)](https://central.sonatype.com/artifact/org.pastalab.fray.maven/fray-plugins-maven)
 
 
 Fray is a concurrency testing tool for Java that can help you **find and debug tricky race conditions that manifest as assertion violations, run-time exceptions, or deadlocks**.