Add note about PERL6LIB being a developer convenience

Thanks to masak++ for pointing this out

lib/Language/5to6.pod

@@ -1073,6 +1073,12 @@ library path within the program via the C<use lib> pragma:
 
     use lib '/some/module/lib'
 
+Note that C<PERL6LIB> is more of a developer convenience in Perl6 (as
+opposed to the equivalent usage of C<PERL5LIB> in Perl5) and shouldn't be
+used by module consumers as it could be removed in the future.  This is
+because Perl6's module loading isn't directly compatible with operating
+system paths.
+
 =head2 Misc.
 
 =head3 C<dump>