Add Maven Central badges

[Goooler]

Overview

I hereby agree to the terms of the JUnit Contributor License Agreement.

---

Definition of Done

• There are no TODOs left in the code
• Method preconditions are checked and documented in the method's Javadoc
• Coding conventions (e.g. for logging) have been followed
• Change is covered by automated tests including corner cases, errors, and exception handling
• Public API has Javadoc and @API annotations
• Change is documented in the User Guide and Release Notes

[marcphilipp]

Team decision: Please use the same badges as we use on junit.org:

junit5/index.html

Lines 42 to 50 in 6d1acba

	<a href="https://central.sonatype.com/search?namespace=org.junit.jupiter">
	<img alt="JUnit Jupiter version" src="https://img.shields.io/maven-central/v/org.junit.jupiter/junit-jupiter/5..svg?color=25a162&label=Jupiter">
	</a>
	<a href="https://central.sonatype.com/search?namespace=org.junit.vintage">
	<img alt="JUnit Vintage version" src="https://img.shields.io/maven-central/v/org.junit.vintage/junit-vintage-engine/5..svg?color=25a162&label=Vintage">
	</a>
	<a href="https://central.sonatype.com/search?namespace=org.junit.platform">
	<img alt="JUnit Platform version" src="https://img.shields.io/maven-central/v/org.junit.platform/junit-platform-commons/1..svg?color=25a162&label=Platform">
	</a>

[marcphilipp]

Could you please convert them to Markdown syntax?

[marcphilipp]

@Goooler Thanks! 👍