docs: add all mvn badges (#115)
docs: add all mvn badges (#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)
GoogleCloudPlatform/functions-framework-java,GoogleCloudPlatform/functions-framework-java