bnd-maven-plugin: Clarify property handling

[kwin]

According to

bnd/maven/bnd-maven-plugin/src/main/java/aQute/bnd/maven/plugin/AbstractBndMavenPlugin.java

Lines 225 to 235 in 7989c55

	Properties beanProperties = new BeanProperties();
	beanProperties.put("project", project);
	beanProperties.put("settings", settings);
	Properties mavenProperties = new Properties(beanProperties);
	Properties projectProperties = project.getProperties();
	for (Enumeration<?> propertyNames = projectProperties.propertyNames(); propertyNames.hasMoreElements();) {
	Object key = propertyNames.nextElement();
	mavenProperties.put(key, projectProperties.get(key));
	}
	try (Builder builder = new Builder(new Processor(mavenProperties, false))) {

all Maven properties and apart from that the settings fields and project fields (with prefix settings/project) can be used in bnd files processed by the bnd-maven-plugin. This should be clarified in the README.MD