Try to fix some broken links

see https://github.com/ruby/docs.ruby-lang.org/issues/56 git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63131 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
2018-04-10 15:29:16 +00:00 · 2018-04-10 15:29:16 +00:00 · 342709d187
--- a/.document
+++ b/.document
@ -24,4 +24,8 @@ NEWS
 README.md
 README.ja.md

+COPYING
+COPYING.ja
+CONTRIBUTING.md
+
 doc