add basic b4 config file

b4[1] is a very nice tool for mail-based contribution. A config[2] file
exists to set up a few defaults. We can use it to set the To recipients
to always add, in our case the mailing list.

This shouldn't be necessary if we had a script that b4 prep --auto-to-cc
could call to find the mail address(es) to send to. Let's start without
it for now.

[1] https://pypi.org/project/b4/
[2] https://b4.docs.kernel.org/en/latest/config.html

Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Signed-off-by: Jon Mason <jon.mason@arm.com>

.b4-config

@@ -0,0 +1,2 @@
+[b4]
+	send-series-to = meta-arm@lists.yoctoproject.org