doc: improve root-mapping documentation

Added test to verify both type and field can be mapped absolutely

README.md

@@ -94,17 +94,23 @@ Each `@Tag` annotation must include a `path` property, using an XPath-like expre
 
 Path expressions can be **absolute**, starting with a slash, representing a path from the root tag to the mapped tag.
 **Relative** paths, without a starting slash, require a parent to be mapped absolutely.
+Root mappings can be placed top-level on the class, all subsequent relative mappings are relative to the root mapping. 
 
 ```java
 import java.math.BigDecimal;
 
+@Tag(path = "/WeatherData")
 class Weather {
     // not annotated with @Tag hence is ignored
     String id;
 
     // example for an absolute mapped tag
     @Tag(path = "/WeatherData/Location") 
     Location location;
+
+	// example for a relative-mapped tag based upon the top-level mapping
+	@Tag(path = "CurrentConditions")
+	Conditions conditions;
 
     // normally all fields without @Tag are ignored, yet this field is 
     // taken into account because at least one of its child fields is
@@ -130,6 +136,16 @@ class Temperature {
 }
 ```
 
+Absolute mapping a field of top-level class containing a top-level root mapping is supported.
+```java
+@Tag(path = "/WeatherData")
+class Weather {
+    @Tag(path = "/WeatherData/Location")
+    Location location;
+}
+
+```
+
 ### Attributes
 
 

xjx-serdes/src/test/java/io/jonasg/xjx/serdes/deserialize/GeneralDeserializationTest.java

@@ -216,4 +216,27 @@ static class WhiteSpacePathMappingHolder {
         @Tag(path = "   /DataTypes/Double   ")
         Double Double;
     }
+
+	@Test
+	void absoluteRootMappingWithTopLevelMappedRootType() {
+		// given
+		String data = """
+				<?xml version="1.0" encoding="UTF-8"?>
+				<DataTypes>
+					<Double>5.7</Double>
+				</DataTypes>
+				""";
+
+		// when
+		var holder = new XjxSerdes().read(data, AbsoluteRootMappingHolder.class);
+
+		// then
+		assertThat(holder.Double).isEqualTo(5.7D);
+	}
+
+	@Tag(path = "/DataTypes")
+	static class AbsoluteRootMappingHolder {
+		@Tag(path = "/DataTypes/Double")
+		Double Double;
+	}
 }