mention default value processing in the README

Author: martin-koch

README.md

@@ -300,6 +300,57 @@ Supported formats: `date-time, date, time, email, hostname, ipv4, ipv6, uuid, re
 
 More formats can be added in `src/string-format-check.cpp`. Please contribute implementions for missing json schema draft formats. 
 
+## Default value processing
+As a result of the validation, the library returns a json patch including the default values of the specified schema.
+
+```C++
+#include <iostream>
+#include <nlohmann/json-schema.hpp>
+
+using nlohmann::json;
+using nlohmann::json_schema::json_validator;
+
+static const json rectangle_schema = R"(
+{
+    "$schema": "http://json-schema.org/draft-07/schema#",
+    "title": "A rectangle",
+    "properties": {
+        "width": {
+            "$ref": "#/definitions/length",
+            "default": 20
+        },
+        "height": {
+            "$ref": "#/definitions/length"
+        }
+    },
+    "definitions": {
+        "length": {
+            "type": "integer",
+            "minimum": 1,
+            "default": 10
+        }
+    }
+})"_json;
+
+int main()
+{
+	try {
+		json_validator validator{rectangle_schema};
+		/* validate empty json -> will be expanded by the default values defined in the schema */
+		json rectangle = "{}"_json;
+		const auto default_patch = validator.validate(rectangle);
+		rectangle = rectangle.patch(default_patch);
+		std::cout << rectangle.dump() << std::endl; // {"height":10,"width":20}
+	} catch (const std::exception &e) {
+		std::cerr << "Validation of schema failed: " << e.what() << "\n";
+		return EXIT_FAILURE;
+	}
+	return EXIT_SUCCESS;
+}
+```
+The example above will output the specified default values `{"height":10,"width":20}` to stdout.
+> Note that the default value specified in a `$ref` may be overridden by the current instance location. Also note that this behavior will break draft-7, but it is compliant to newer drafts (e.g. `2019-09` or `2020-12`).
+
 # Contributing
 
 Before opening a pull request, please apply the coding style given in the