Add Maven Central badges

[Goooler]

Overview

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

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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:

https://github.com/junit-team/junit5/blob/6d1acba62883bf98aee228c50c693569466c9fbe/index.html#L42-L50

[marcphilipp]

Could you please convert them to Markdown syntax?

[marcphilipp]

@Goooler Thanks! 👍