docs: add all mvn badges (GoogleCloudPlatform#115)

Add badges to all published packages from this repo.

[![Maven Central](https://img.shields.io/maven-central/v/com.google.cloud.functions/functions-framework-api.svg?label=functions-framework-api)](https://search.maven.org/artifact/com.google.cloud.functions/functions-framework-api)
[![Maven Central](https://img.shields.io/maven-central/v/com.google.cloud.functions.invoker/java-function-invoker.svg?label=java-function-invoker)](https://search.maven.org/artifact/com.google.cloud.functions.invoker/java-function-invoker)
[![Maven Central](https://img.shields.io/maven-central/v/com.google.cloud.functions/function-maven-plugin.svg?label=function-maven-plugin)](https://search.maven.org/artifact/com.google.cloud.functions/function-maven-plugin)

Author: grant

README.md

@@ -1,7 +1,8 @@
 # Functions Framework for Java
 
-[![Maven Central](https://img.shields.io/maven-central/v/com.google.cloud.functions/functions-framework-api.svg?label=functions-framework-api)](https://search.maven.org/artifact/com.google.cloud.functions/functions-framework-api)
-[![Maven Central](https://img.shields.io/maven-central/v/com.google.cloud.functions.invoker/java-function-invoker.svg?label=java-function-invoker)](https://search.maven.org/artifact/com.google.cloud.functions.invoker/java-function-invoker)
+[![Maven Central (functions-framework-api)](https://img.shields.io/maven-central/v/com.google.cloud.functions/functions-framework-api.svg?label=functions-framework-api)](https://search.maven.org/artifact/com.google.cloud.functions/functions-framework-api)
+[![Maven Central (java-function-invoker)](https://img.shields.io/maven-central/v/com.google.cloud.functions.invoker/java-function-invoker.svg?label=java-function-invoker)](https://search.maven.org/artifact/com.google.cloud.functions.invoker/java-function-invoker)
+[![Maven Central (function-maven-plugin)](https://img.shields.io/maven-central/v/com.google.cloud.functions/function-maven-plugin.svg?label=function-maven-plugin)](https://search.maven.org/artifact/com.google.cloud.functions/function-maven-plugin)
 
 [![Java Unit CI](https://github.com/GoogleCloudPlatform/functions-framework-java/actions/workflows/unit.yaml/badge.svg)](https://github.com/GoogleCloudPlatform/functions-framework-java/actions/workflows/unit.yaml)
 [![Java Lint CI](https://github.com/GoogleCloudPlatform/functions-framework-java/actions/workflows/lint.yaml/badge.svg)](https://github.com/GoogleCloudPlatform/functions-framework-java/actions/workflows/lint.yaml)