Add example for SetHash initialization.

See issue #1939

doc/Type/SetHash.pod6

@@ -66,6 +66,15 @@ the set - and keys mapped to values which boolify to C<False> are skipped:
     say $n.keys.perl;        # OUTPUT: «("one", "two").Seq␤»
     say $n.keys.map(&WHAT);  # OUTPUT: «((Str) (Str))␤»
 
+It is also possible to initialize a single key with the use of C<{}>:
+
+    my $sh = SetHash.new;
+    $sh{ 'key1' } = True;
+    $sh{ 'key2' } = 'Hello World!';
+    $sh{ 'key3' } = 0;  # does not store the key since 0.Bool is False
+    say $sh;            # OUTPUT: «SetHash(key1 key2)␤»
+    say $sh.keys.perl;  # OUTPUT: «("key1", "key2").Seq␤»
+
 =head1 Operators
 
 Perl 6 provides common set operators, which can take C<SetHash>es (or any other