Add description of Signed-off-by lines

dtc and libfdt have been using Signed-off-by lines (as used in the Linux
kernel) for some time, like a lot of open source projects.  However
Uwe Kleine-König pointed out we never really stated what they mean in our
context.

Add information on what the S-o-b line means in CONTRIBUTING.md - this is
essentially a quote of the same information from the kernel documentation,
with some tweaks to make sense in the new context.

Suggested-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>

CONTRIBUTING.md

@@ -23,3 +23,57 @@ all tests.
 If you want to adjust a test file, be aware that `tree_tree1.dts` is compiled
 and checked against a binary tree from assembler macros in `trees.S`. So
 if you change that file you must change `tree.S` also.
+
+## Developer's Certificate of Origin
+
+Like many other projects, dtc and libfdt have adopted the "Developer's
+Certificate of Origin" (Signed-off-by) process created by the Linux
+kernel community to improve tracking of who did what.  Here's how it
+works (this is a very slight modification of the description from
+`Documentation/process/submitting-patches.rst` in the kernel tree):
+
+The sign-off is a simple line at the end of the explanation for the
+patch, which certifies that you wrote it or otherwise have the right
+to pass it on as an open-source patch.  The rules are pretty simple:
+if you can certify the below:
+
+    Developer's Certificate of Origin 1.1
+
+    By making a contribution to this project, I certify that:
+
+        (a) The contribution was created in whole or in part by me and I
+            have the right to submit it under the open source license
+            indicated in the file; or
+
+        (b) The contribution is based upon previous work that, to the best
+            of my knowledge, is covered under an appropriate open source
+            license and I have the right under that license to submit that
+            work with modifications, whether created in whole or in part
+            by me, under the same open source license (unless I am
+            permitted to submit under a different license), as indicated
+            in the file; or
+
+        (c) The contribution was provided directly to me by some other
+            person who certified (a), (b) or (c) and I have not modified
+            it.
+
+        (d) I understand and agree that this project and the contribution
+            are public and that a record of the contribution (including all
+            personal information I submit with it, including my sign-off) is
+            maintained indefinitely and may be redistributed consistent with
+            this project or the open source license(s) involved.
+
+then you just add a line saying::
+
+	Signed-off-by: Random J Developer <random@developer.example.org>
+
+using your real name (sorry, no pseudonyms or anonymous
+contributions.)  This will be done for you automatically if you use
+`git commit -s`.  Reverts should also include "Signed-off-by". `git
+revert -s` does that for you.
+
+Any further SoBs (Signed-off-by:'s) following the author's SoB are
+from people handling and transporting the patch, but were not involved
+in its development. SoB chains should reflect the **real** route a
+patch took as it was propagated to the maintainers, with the first SoB
+entry signalling primary authorship of a single author.