Document how to use aviz.

Author: Crell

spec.md

@@ -659,12 +659,13 @@ engine and language handles them. When present, they MUST be in the following or
 
 * Inheritance modifier: `abstract` or `final`
 * Visibility modifier: `public`, `protected`, or `private`
+* Set-visibility modifier: `public(set)`, `protected(set)`, or `private(set)`
 * Scope modifier: `static`
 * Mutation modifier: `readonly`
 * Type declaration
 * Name
 
-All keywords MUST be on a single line, and MUST be separated by a single space.
+All keywords MUST be on a single line, and MUST be separated by a single space.  All keywords MUST be all lower-case.  When using a set-visibility on a public-read property, omitting the `public` keyword is RECOMMENDED but NOT REQUIRED.
 
 The following is a correct example of modifier keyword usage:
 
@@ -679,6 +680,10 @@ abstract class ClassName
 
     private readonly int $beep;
 
+    protected private(set) string $name;
+
+    protected(set) string $boop;
+
     abstract protected function zim();
 
     final public static function bar()