doc: Don't vendor a copy of DCO, as it is under another license.

Closes #36 as merged as of commit d242104.

Signed-off-by: Simon Josefsson <simon@josefsson.org>
Change-Id: I03e9cd13c413d26fab1d3eaf3650645d33159ba0
Reviewed-on: https://review.gerrithub.io/c/cue-labs/oci/+/1204556
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: CUE porcuepine <cue.porcuepine@gmail.com>

Author: jas4711

CONTRIBUTING.md

@@ -57,48 +57,7 @@ of Origin.
 ### Asserting a Developer Certificate of Origin
 
 Contributions must be accompanied by a [Developer Certificate of
-Origin](https://developercertificate.org/), the text of which is reproduced here
-for convenience:
-
-```
-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.
-```
+Origin](https://developercertificate.org/), we are using version 1.1.
 
 All commit messages must contain the `Signed-off-by` line with an email address
 that matches the commit author. This line asserts the Developer Certificate of Origin.