Add se section about P5built-ins

doc/Language/5to6-nutshell.pod6

@@ -53,6 +53,14 @@ A number of Perl 5 modules have been ported to Perl 6, trying to maintain
 the API of these modules as much as possible, as part of the CPAN Butterfly
 Plan.  These can be found at L<https://modules.perl6.org/t/CPAN5>.
 
+Many Perl 5 built-in functions (about a 100 so far) have been ported to
+Perl 6 with the same semantics.  Think about the C<shift> function in Perl 5
+having magic shifting from C<@_> or C<@ARGV> by default, depending on context.
+These can be found at L<https://modules.perl6.org/t/Perl5> as separately
+loadable moduels, and in the
+L<P5built-ins|https://modules.perl6.org/dist/P5built-ins> bundle to get them
+all at once..
+
 =head1 Syntax
 
 =head2 Identifiers