README: create HTTP/HTTPS links from URLs in Markdown

Markdown supports automatic links by surrounding URLs with
angle brackets, as documented in
<https://daringfireball.net/projects/markdown/syntax#autolink

>

While we're at it, update URLs to avoid redirecting clients for
git-scm.com (by using HTTPS) and public-inbox.org (by adding a
trailing slash).

Signed-off-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>