Mention build-time attribute defaults in classtut.

Closes #79

lib/Language/classtut.pod

@@ -166,6 +166,17 @@ X<|traits, is rw>
 The C<is rw> trait causes the generated accessor method to return something
 external code can modify to change the value of the attribute.
 
+You can also supply default values to attributes (which works equally for
+those with and without accessors):
+
+    has Bool $.done = False;
+
+The assignment is carried out at object build time. The right-hand side is
+evaluated at that time, and can even reference earlier attributes:
+
+    has Task @!dependencies;
+    has $.ready = not @!dependencies;
+
 =head1 Static fields?
 
 Perl 6 has no B<static> keyword. Nevertheless any class may declare anything
@@ -304,6 +315,22 @@ as the bind targets for C<BUILD>'s parameters. Zero-boilerplate
 initialization! See L<objects|/language/objects#Object Construction> for
 more information.
 
+The C<BUILD> method is responsible for initialilzing all attributes, it must
+handle default values too:
+
+    has &!callback;
+    has @!dependencies;
+    has Bool ($.done, $.ready);
+    submethod BUILD(
+            :@!callback,
+            :@!dependencies,
+            :$!done = False,
+            :$!ready = not @!dependencies
+        ) { }
+
+See L<Object Construction|/language/objects#Object Construction> for more
+options to influence object construction and attribute initializtion.
+
 =head1 Consuming our class
 
 After creating a class, you can create instances of the class.  Declaring a