[variables] document the ? twigil

lib/variables.pod

@@ -120,6 +120,23 @@ private and can be directly accessed within the class by using
 C<$!attribute-name>. Perl 6 may automatically generate accessor methods for you
 tough. For more details on objects, classes and their attributes see L<objects>.
 
+=head2 ?
+
+Compile-time "constants" may be addressed via the C<?> twigil. They are known to
+the compiler and may not be modified after being compiled in. A popular example
+for this is:
+
+    say "$?FILE: $?LINE"; # prints "hello.pl: 23" if this is the 23 line of a
+                          # file named "hello.pl".
+
+Although they may not be changed at runtime, the user is allowed to (re)define
+such constants.
+
+    constant $?TABSTOP = 4; # this causes leading tabs in a heredoc or in a POD
+                            # block's virtual margin to be counted as 4 spaces.
+
+For a list of those special variables see L<Compile-time "constants">.
+
 =head1 Special Variables
 
 =head2 Often-Used Variables