patman: check git format.subjectprefix setting when generate patches prefix

For the local project, we may specified format.subjectprefix setting.
Then the patch will be formated as [Project_prefix][PATCH].
But patman will not check this setting. It will remove the
format.subjectprefix.

So This patch will let patman check this setting and add it as a
project prefix.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Acked-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/patman/README b/tools/patman/README
index 7d039e8..27ec90a 100644
--- a/tools/patman/README
+++ b/tools/patman/README
@@ -154,7 +154,11 @@
 
 Series-prefix: prefix
 	Sets the subject prefix. Normally empty but it can be RFC for
-	RFC patches, or RESEND if you are being ignored.
+	RFC patches, or RESEND if you are being ignored. The patch subject
+	is like [RFC PATCH] or [RESEND PATCH].
+	In the meantime, git format.subjectprefix option will be added as
+	well. If your format.subjectprefix is set to InternalProject, then
+	the patch shows like: [InternalProject][RFC/RESEND PATCH]
 
 Series-name: name
 	Sets the name of the series. You don't need to have a name, and