Include copyright notice, contributors, and license in the deb package

MXNet includes a separate `NOTICE` file for its copyright declaration,
together with the `LICENSE` file containing the Apache License v2.0.
This patch updates the deb package definition to make sure that both are
included in `/usr/share/doc/libmxnet/`.  Since the copyright declaration
only refers to 'Contributors', the contributor list is also included.

Note that in principle the `copyright` file should have a fairly strict
format [1], but this is left for a future package update, since there is
no strong need at present for this file to be machine-readable.

[1] https://www.debian.org/doc/packaging-manuals/copyright-format/1.0/

pkg/libmxnet.pkg

@@ -21,6 +21,8 @@ It has been built with OpenCV and OpenMP support disabled.
 doc_install_dir = '/usr/share/doc/' + VAR.fullname + '/'
 
 ARGS = FUN.mapfiles('./lib', '/usr/lib', libs, append_suffix=False) + [
-    'LICENSE=' + doc_install_dir + 'copyright',
+    'NOTICE=' + doc_install_dir + 'copyright',
+    'LICENSE=' + doc_install_dir,
+    'CONTRIBUTORS.md=' + doc_install_dir,
     'README.md=' + doc_install_dir,
 ]