README: Update links to mailing list archives

This patch includes these updates

- Change http://lists.denx.de/pipermail/u-boot
  to https://lists.denx.de/pipermail/u-boot
- http://dir.gmane.org/gmane.comp.boot-loaders.u-boot
  is broken, so remove it
- Another archive https://marc.info/?l=u-boot
  is available, so add it

Additional information:
dir.gmane.org has moved to news.gmane.io, but it only allows
access to the archive via NNTP.

- Server: news.gmane.io
- Group : gmane.comp.boot-loaders.u-boot

I don't include this information in this patch because
it's not very useful without web interfaces.

Signed-off-by: Naoki Hayama <naoki.hayama@lineo.co.jp>

README

@@ -51,9 +51,8 @@ In case you have questions about, problems with or contributions for
 U-Boot, you should send a message to the U-Boot mailing list at
 <u-boot@lists.denx.de>. There is also an archive of previous traffic
 on the mailing list - please search the archive before asking FAQ's.
-Please see http://lists.denx.de/pipermail/u-boot and
-http://dir.gmane.org/gmane.comp.boot-loaders.u-boot
-
+Please see https://lists.denx.de/pipermail/u-boot and
+https://marc.info/?l=u-boot
 
 Where to get source code:
 =========================