Update README with link to documentation and explicitly state the Java 17+ requirement (#268)

Author: rbygrave

README.md

@@ -20,6 +20,14 @@ Reflection-free pojo validation via apt source code generation. A light (~120kb
 - Getter Validation
 - Method parameter validation (requires a DI container to retrieve the generated MethodAdapter classes)
 
+# Documentation
+
+Goto [https://avaje.io/validator/](https://avaje.io/validator/) for full documentation.
+
+# Requires Java 17+
+
+Avaje Validator requires Java 17 or higher.
+
 # Quick Start
 
 ## Step 1 - Add dependencies