Documentation/technical: signed commit format

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/signature-format.txt

@@ -62,3 +62,51 @@ signed tag
 
 signed tag message body
 ----
+
+== Commit signatures
+
+- created by: `git commit -S`
+- payload: commit object
+- embedding: header entry `gpgsig`
+  (content is preceded by a space)
+- example: commit with subject `signed commit`
+
+----
+tree eebfed94e75e7760540d1485c740902590a00332
+parent 04b871796dc0420f8e7561a895b52484b701d51a
+author A U Thor <author@example.com> 1465981137 +0000
+committer C O Mitter <committer@example.com> 1465981137 +0000
+gpgsig -----BEGIN PGP SIGNATURE-----
+ Version: GnuPG v1
+
+ iQEcBAABAgAGBQJXYRjRAAoJEGEJLoW3InGJ3IwIAIY4SA6GxY3BjL60YyvsJPh/
+ HRCJwH+w7wt3Yc/9/bW2F+gF72kdHOOs2jfv+OZhq0q4OAN6fvVSczISY/82LpS7
+ DVdMQj2/YcHDT4xrDNBnXnviDO9G7am/9OE77kEbXrp7QPxvhjkicHNwy2rEflAA
+ zn075rtEERDHr8nRYiDh8eVrefSO7D+bdQ7gv+7GsYMsd2auJWi1dHOSfTr9HIF4
+ HJhWXT9d2f8W+diRYXGh4X0wYiGg6na/soXc+vdtDYBzIxanRqjg8jCAeo1eOTk1
+ EdTwhcTZlI0x5pvJ3H0+4hA2jtldVtmPM4OTB0cTrEWBad7XV6YgiyuII73Ve3I=
+ =jKHM
+ -----END PGP SIGNATURE-----
+
+signed commit
+
+signed commit message body
+----
+
+- verify with: `git verify-commit [-v]` (or `git show --show-signature`)
+
+----
+gpg: Signature made Wed Jun 15 10:58:57 2016 CEST using RSA key ID B7227189
+gpg: Good signature from "Eris Discordia <discord@example.net>"
+gpg: WARNING: This key is not certified with a trusted signature!
+gpg:          There is no indication that the signature belongs to the owner.
+Primary key fingerprint: D4BE 2231 1AD3 131E 5EDA  29A4 6109 2E85 B722 7189
+tree eebfed94e75e7760540d1485c740902590a00332
+parent 04b871796dc0420f8e7561a895b52484b701d51a
+author A U Thor <author@example.com> 1465981137 +0000
+committer C O Mitter <committer@example.com> 1465981137 +0000
+
+signed commit
+
+signed commit message body
+----