Some links broken in README

[willnet]

Links of COPYING, and CONTRIBUTING.md seem to be broken in https://docs.ruby-lang.org/en/trunk/ . They return 404.

I'm not sure that this Issue related to here. sorry if it's wrong.

[znz]

win32/README.win32 too.

[znz]

ref: Some links broken in README

[hsbt]

Fixed by https://bugs.ruby-lang.org/projects/ruby-master/repository/git/revisions/2898805535104c330e1e5c64c8065a108f1f2895/diff/README.md