doc: Update README with UUID usage examples (#263)

README.md

@@ -282,4 +282,47 @@ DB.Where(datatypes.JSONArrayQuery("config").Contains("c")).Find(&retMultiple)
 }
 ```
 
+## UUID
 
+MySQL, PostgreSQL, SQLServer and SQLite are supported.
+
+```go
+import "gorm.io/datatypes"
+
+type UserWithUUID struct {
+    gorm.Model
+    Name string
+    UserUUID datatypes.UUID
+}
+
+// Generate a new random UUID (version 4).
+userUUID := datatypes.NewUUIDv4()
+
+user := UserWithUUID{Name: "jinzhu", UserUUID: userUUID}
+DB.Create(&user)
+// INSERT INTO `user_with_uuids` (`name`,`user_uuid`) VALUES ("jinzhu","ca95a578-816c-4812-babd-a7602b042460")
+
+var result UserWithUUID
+DB.First(&result, "name = ? AND user_uuid = ?", "jinzhu", userUUID)
+// SELECT * FROM user_with_uuids WHERE name = "jinzhu" AND user_uuid = "ca95a578-816c-4812-babd-a7602b042460" ORDER BY `user_with_uuids`.`id` LIMIT 1
+
+// Use the datatype's Equals() to compare the UUIDs.
+if userCreate.UserUUID.Equals(userFound.UserUUID) {
+	fmt.Println("User UUIDs match as expected.")
+} else {
+	fmt.Println("User UUIDs do not match. Something is wrong.")
+}
+
+// Use the datatype's String() function to get the UUID as a string type.
+fmt.Printf("User UUID is %s", userFound.UserUUID.String())
+
+// Check the UUID value with datatype's IsNil() and IsEmpty() functions.
+if userFound.UserUUID.IsNil() {
+	fmt.Println("User UUID is a nil UUID (i.e. all bits are zero)")
+}
+if userFound.UserUUID.IsEmpty() {
+	fmt.Println(
+		"User UUID is empty (i.e. either a nil UUID or a zero length string)",
+	)
+}
+```