Describe how to add extra mail header lines in mail generated by git-format-patch.

Documentation/git-format-patch.txt

diff --git a/Documentation/git-format-patch.txt b/Documentation/git-format-patch.txt
index bc85a62..7cc7faf 100644
--- a/Documentation/git-format-patch.txt
+++ b/Documentation/git-format-patch.txt
@@ -64,6 +64,15 @@
 	Create attachments instead of inlining patches.
 
 
+CONFIGURATION
+-------------
+You can specify extra mail header lines to be added to each
+message in the repository configuration as follows:
+
+[format]
+        headers = "Organization: git-foo\n"
+
+
 EXAMPLES
 --------