docs: add badges for Maven Central and CI (#112)
docs: add badges for Maven Central and CI (#112)

Adds detailed badges/links to main packages and CI.

[![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)

[![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)
[![Java Conformance CI](https://github.com/GoogleCloudPlatform/functions-framework-java/actions/workflows/conformance.yaml/badge.svg)](https://github.com/GoogleCloudPlatform/functions-framework-java/actions/workflows/conformance.yaml)
GoogleCloudPlatform/functions-framework-java,GoogleCloudPlatform/functions-framework-java