explain 3 vs 4

Author: hboutemy

README.md

@@ -2,11 +2,18 @@ Plexus-Xml
 ============
 
 [![Build Status](https://github.com/codehaus-plexus/plexus-xml/actions/workflows/maven.yml/badge.svg)](https://github.com/codehaus-plexus/plexus-xml/actions)
+[![Maven Central](https://img.shields.io/maven-central/v/org.codehaus.plexus/plexus-xml.svg?label=Maven%20Central&versionPrefix=3.)](https://search.maven.org/artifact/org.codehaus.plexus/plexus-xml)
 [![Maven Central](https://img.shields.io/maven-central/v/org.codehaus.plexus/plexus-xml.svg?label=Maven%20Central)](https://search.maven.org/artifact/org.codehaus.plexus/plexus-xml)
 
 This library consists of XML classes (`org.codehaus.plexus.util.xml` and `org.codehaus.plexus.util.xml.pull`) that:
-1. have been extracted from [`plexus-utils`](https://github.com/codehaus-plexus/plexus-utils/) 4,
-2. then updated to use [`maven-xml-api`](https://github.com/apache/maven/tree/maven-4.0.0-alpha-7/api/maven-api-xml)/[`maven-xml-impl`](https://github.com/apache/maven/tree/maven-4.0.0-alpha-7/maven-xml-impl)
+
+1. have been extracted from [`plexus-utils`](https://github.com/codehaus-plexus/plexus-utils/) 4:\
+   this step is released as `plexus-xml` 3, maintained in [3.x branch](tree/3.x)\
+   [![Maven Central](https://img.shields.io/maven-central/v/org.codehaus.plexus/plexus-xml.svg?label=Maven%20Central&versionPrefix=3.)](https://search.maven.org/artifact/org.codehaus.plexus/plexus-xml),
+
+2. then updated to use [`maven-xml-api`](https://github.com/apache/maven/tree/maven-4.0.0-alpha-7/api/maven-api-xml)/[`maven-xml-impl`](https://github.com/apache/maven/tree/maven-4.0.0-alpha-7/maven-xml-impl):\
+   this is the `master` branch from which `plexus-xml` 4 is released\
+   [![Maven Central](https://img.shields.io/maven-central/v/org.codehaus.plexus/plexus-xml.svg?label=Maven%20Central)](https://search.maven.org/artifact/org.codehaus.plexus/plexus-xml)
 
 For publishing [the site](https://codehaus-plexus.github.io/plexus-xml/) do the following: