Add in maven-central badge

[ckipp01]

This a small pr that just adds in a maven central badge to pull in the latest version and display it on the readme.

Whenever I check latest versions I always default to checking the github page. This just saves me a step to not have to click into the site and find it there.

[olafurpg]

👍 I understand this may not be an ideal workflow for many but I use cs complete

❯ cs complete org.scalameta:mdoc_2.13:
https://repo1.maven.org/maven2/org/scalameta/mdoc_2.13/maven-metadata.xml
  100.0% [##########] 802 B (11.0 KiB / s)
1.3.2
1.3.4
1.3.5
1.3.6
1.4.0-RC2
1.4.0-RC3
2.0.0
2.0.1
2.0.2
2.0.3
2.1.0
2.1.1
2.1.2
2.1.3
2.1.4
2.1.5

[olafurpg]

It works to complete both organizations and module names, really handy to explore the cross-build of a library

[ckipp01]

👍 I understand this may not be an ideal workflow for many but I use cs complete

❯ cs complete org.scalameta:mdoc_2.13:
https://repo1.maven.org/maven2/org/scalameta/mdoc_2.13/maven-metadata.xml
  100.0% [##########] 802 B (11.0 KiB / s)
1.3.2
1.3.4
1.3.5
1.3.6
1.4.0-RC2
1.4.0-RC3
2.0.0
2.0.1
2.0.2
2.0.3
2.1.0
2.1.1
2.1.2
2.1.3
2.1.4
2.1.5

oooooo, I've actually not used this command before. That's incredible useful for me. Thanks for showing me this!