An example showing how to set PHP-namespace in PHP-CPP

PHP-CPP makes it possible very easy to define a PHP-namespace for your classes and functions.
To do this, just need to add the prefix **`NamespaceExample\`** to the name of a class or function:
```c
        ...
        extension.add(
            "NamespaceExample\\TestClass", 
            Php::Class<MyTestClass>({
                Php::Public("myMethod", Php::Method<MyTestClass>(&MyTestClass::myMethod)),
            })
        );

        extension.add("NamespaceExample\\TestFunc", MyTestFunc);
        ...
```
Explanation of why this is so see [README](https://github.com/valmat/PHP-CPP/blob/namespace-example/Examples/Namespaces/README.md)


This Pull Request, I suggest example, documenting this opportunity.