contributing: add copy of DCO

The DCO was fetched from https://developercertificate.org/ on Feb 4, 2019.
DCO-1.1.txt is a verbatim copy of it.

We should carry a copy of this legal document to be explicit on what wording we
refer to. A link to the URL is not enough, because it may go stale or change
contents without notice.

This clarifies on what the S-o-b actually means. The meaning itself is the same
as before.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

CONTRIBUTING.md

@@ -98,7 +98,9 @@ See [notes on commit messages] for a recommended reading on writing commit
 messages.
 
 Your patches should also include a Signed-off-by line with your name and
-email address.  If you're not the patch's original author, you should
+email address which indicates that you agree to the
+[Developer's Certificate of Origin 1.1](DCO-1.1.txt).
+If you're not the patch's original author, you should
 also gather S-o-b's by them (and/or whomever gave the patch to you.) The
 significance of this is that it certifies that you created the patch,
 that it was created under an appropriate open source license, or

DCO-1.1.txt

@@ -0,0 +1,37 @@
+Developer Certificate of Origin
+Version 1.1
+
+Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
+1 Letterman Drive
+Suite D4700
+San Francisco, CA, 94129
+
+Everyone is permitted to copy and distribute verbatim copies of this
+license document, but changing it is not allowed.
+
+
+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.