patman: Add "postfix" support to patch subjects

In some communities, it may be necessary to append something after PATCH
in the subject line. For example, the Linux networking subsystem
expects [1] patch subject prefixes like [RFC PATCH net-next 0/99]. This
adds support for such "postfix"s to patman. Although entirely cosmetic,
it is still nice to have.

[1] https://www.kernel.org/doc/html/latest/networking/netdev-FAQ.html#how-do-i-indicate-which-tree-net-vs-net-next-my-patch-should-be-in

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/patman/README b/tools/patman/README
index 53f55ce..e3466e6 100644
--- a/tools/patman/README
+++ b/tools/patman/README
@@ -188,6 +188,11 @@
 	well. If your format.subjectprefix is set to InternalProject, then
 	the patch shows like: [InternalProject][RFC/RESEND PATCH]
 
+Series-postfix: postfix
+	Sets the subject "postfix". Normally empty, but can be the name of a
+	tree such as net or net-next if that needs to be specified. The patch
+	subject is like [PATCH net] or [PATCH net-next].
+
 Series-name: name
 	Sets the name of the series. You don't need to have a name, and
 	patman does not yet use it, but it is convenient to put the branch