add a reference to Build.pm6 for zef installation

Author: tbrowder

doc/Language/modules.pod6

@@ -659,6 +659,12 @@ appear next to your module at the module directory, create a
 C<logotype> directory and put into it a C<logo_32x32.png> file. At some
 point, you might also consider adding C<CONTRIBUTORS>, C<NEWS>,
 C<TODO>, or other files.>
+
+If your module requires extra processing during installation to fully
+integrate with and use non-Perl operating system resources, you may
+need to add a C<Build.pm6> file to the top-level directory. It will
+bu used by the C<zef> installer as the first step in the installation process.
+See various usage scenarios in existing ecosystem modules such as C<zef> itself.
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