Mention that smartmatch is deprecated

Author: book

lib/feature.pm

@@ -493,7 +493,7 @@ sub longest {
 __END__
 package feature;
 
-our $VERSION = '1.76';
+our $VERSION = '1.77';
 
 FEATURES
 
@@ -597,6 +597,8 @@ =head2 The 'switch' feature
 See L<perlsyn/"Switch Statements"> for details.
 
 This feature is available starting with Perl 5.10.
+It is deprecated starting with Perl 5.038, and using
+C<given>, C<when> or smartmatch will throw a warning.
 
 =head2 The 'unicode_strings' feature