Add file describing Signed-off-by and guiding new module authors.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

PATCHES

+- To create a new module, just put it under ccan/ and run ccanlint on
+  it; it's designed to tell you what to improve (and you can ignore it!).
+
+- When you're happy, Sign off your commit as detailed below, send to
+  the mailing list as a patch (https://lists.ozlabs.org/listinfo/ccan )
+
+- We'll ask for your ssh key so you can push updates to your module(s).
+
+Cheers!
+The CCAN Maintainers.
+
+(Taken from Linux Kernel's Documentation/SubmittingPatches:)
+
+To improve tracking of who did what, especially with patches that can
+percolate to their final resting place in the kernel through several
+layers of maintainers, we've introduced a "sign-off" procedure on
+patches that are being emailed around.
+
+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>