Adds doc on class attributes, closes #2089

Author: JJ

doc/Language/classtut.pod6

@@ -142,13 +142,11 @@ You can also use type smileys to only accept instances or type objects:
     say foo Int; # OUTPUT: «It's a type object!␤»
     say foo 42;  # OUTPUT: «It's an instance!␤»
 
-=head1 State
-
 X<|attributes>
 X<|classes,attributes>
-
 X<|encapsulation>
 X<|classes,encapsulation>
+=head1 State
 
 The first three lines inside the class block all declare attributes (called
 I<fields> or I<instance storage> in other languages). Just as a C<my>
@@ -231,6 +229,29 @@ evaluated at that time, and can even reference earlier attributes:
 has Task @!dependencies;
 has $.ready = not @!dependencies;
 
+X<|class variables>
+A class declaration can also include I<class variables>, which are variables that are shared by all instances, and can be used for things like counting the number of instantiations or any other shared state. Class variables use the same syntax as the rest of the attributes, but are declared as C<my> or C<our>, depending on the scope; the second type of variables will be used across the class hierarchy.
+
+=begin code
+class Str-with-ID is Str {
+    my $.counter = 0;
+    has Str $.string;
+    has Int $.ID;
+
+    method new( Str $string ) {
+        self.bless( $:string, ID => $.counter++ )
+    }
+}
+
+say Str-with-ID.new('First').ID;  # OUTPUT: «0»
+say Str-with-ID.new('Second').ID; # OUTPUT: «1»
+=end code
+
+In this case, using C<new> might be the easiest way to initialize the C<$.ID>
+field and increment the value of the counter at the same time. C<new>, through
+C<bless>, will invoke the default C<BUILD>, assigninng the values to their
+properties correctly.
+
 =head1 Static fields?
 
 Perl 6 has no B<static> keyword. Nevertheless, any class may declare anything