Change markdown link style in README, INSTALL, SUPPORT and CONTRIBUTING

Change markdown link style in README, INSTALL, SUPPORT and CONTRIBUTING

Replace [collapsed reference links][] by [shortcut reference links],
in order to to improve the readability of the raw markdown text,
see also commit d8dc853825 (Change CVE link style in CHANGES and NEWS).

[collapsed reference links]:
  https://github.github.com/gfm/#collapsed-reference-link

[shortcut reference links]:
  https://github.github.com/gfm/#shortcut-reference-link

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/13165)