Add an .elpaignore file. (#141)

This file is describe in GNU ELPA's README
(https://git.savannah.gnu.org/cgit/emacs/elpa.git/tree/README)
under the heading "Format".  There, it is stated that the patterns of
files are passed to `tar`'s `-X` (`--exclude-from=`) switch.

This simplifies the `:ignored-files` part of the ELPA package
specification.

The recommendation for adding the file was made on the Emacs Devel
mailing list
(https://lists.gnu.org/archive/html/emacs-devel/2022-12/msg00775.html).

.elpaignore

@@ -0,0 +1,5 @@
+Dockerfile
+.github
+Makefile
+scripts
+stub