Add Sonatype Central badge to README (#11)

Author: tanin47

README.md

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 Embeddable Java Web Framework
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+[![Sonatype Central](https://maven-badges.sml.io/sonatype-central/io.github.tanin47/embeddable-java-web-framework/badge.png?8cb1875)](https://central.sonatype.com/artifact/io.github.tanin47/embeddable-java-web-framework)
+
 Embeddable Java Web Framework (EJWF) is a Java project template for building a website with a tiny footprint. 
 It is suitable for <ins>a sidecar-style website embeddable on a larger JVM system</ins> and a standalone lightweight website.