Fix links in CONTRIBUTING.md
Fix links in CONTRIBUTING.md

CHANGES and NEWS were renamed into CHANGES.md and NEWS.md; this commit
updates the references inside CONTRIBUTING.md to fix broken links.

Reviewed-by: Richard Levitte <5fb523282dd7956571c80524edc2dccfa0bd8234@openssl.org>
Reviewed-by: Matthias St. Pierre <1910231d24f125aceac7850dc92279eee8d52323@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/11634)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl