Update references to Yocto mailing list

The mailing list infrastructure at Yocto project changed (for a good
while now) but we still reference the old address/links. Update them
accordingly.

Signed-off-by: Andrei Gherzan <andrei@gherzan.com>

README.md

@@ -13,7 +13,7 @@ Yocto BSP layer for the Raspberry Pi boards - <http://www.raspberrypi.org/>.
 
 * Git repository web frontend:
   <https://github.com/agherzan/meta-raspberrypi>
-* Mailing list (yocto mailing list): <yocto@yoctoproject.org>
+* Mailing list (yocto mailing list): <yocto@lists.yoctoproject.org>
 * Issues management (Github Issues):
   <https://github.com/agherzan/meta-raspberrypi/issues>
 * Documentation: <http://meta-raspberrypi.readthedocs.io/en/latest/>

docs/contributing.md

@@ -4,8 +4,8 @@
 
 The main communication tool in use is the Yocto Project mailing list:
 
-* <yocto@yoctoproject.org>
-* <https://lists.yoctoproject.org/listinfo/yocto>
+* <yocto@lists.yoctoproject.org>
+* <https://lists.yoctoproject.org/g/yocto>
 
 Feel free to ask any kind of questions but please always prepend your email
 subject with `[meta-raspberrypi]` as this is the global *Yocto* mailing
@@ -84,7 +84,7 @@ sure to use the following formatting for the message subject:
 
 Then, for sending patches to the mailing list, you may use this command:
 
-    git send-email --to yocto@yoctoproject.org <generated patch>
+    git send-email --to yocto@lists.yoctoproject.org <generated patch>
 
 ## GitHub issues