Add Developer Certificate of Origin.

Signed-off-by: Roberto Rosario <roberto.rosario.gonzalez@gmail.com>

CONTRIBUTING.md

@@ -17,7 +17,7 @@ same properties that can trigger the issue and upload that file instead.
 - Add steps that trigger the issue in a **repeatable manner**.
 - **Screenshots** go a long way in helping understand problems.
 - The issue must be related to the code only, do not open issues for problems
-with webservers, cloud providers, etc.
+with deployments, webservers, cloud providers, etc.
 - Do not open issues asking for **support or consulting**.
 
 Code
@@ -40,7 +40,19 @@ following branches:
     are unstable and should not be used in production.
 
 1. Start making your changes in your own separate branch.
-1. Write a test which shows that the bug was fixed or that the feature works as expected.
+1. Write a test which shows that the bug was fixed or that the feature works as
+   expected.
+1. Sign your work. Your signature certifies your submission according to the
+   articles of the [Developer Certificate of Origin](https://gitlab.com/mayan-edms/mayan-edms/blob/master/DCO).
+   The sign-off should be in the form:
+
+    ````
+    Signed-off-by: John Doe <john.doe@example.com>
+    ````
+
+    You must use your real name and email, pseudonyms or anonymous contributions
+    are not allowed. If you set your user.name and user.email git configs, you can
+    sign your commit automatically with git commit -s.
 1. Submit a merge request for your changes.
 
 Feature requests
@@ -64,8 +76,3 @@ Code style
 ----------
 - Refer to the [Development](http://mayan.readthedocs.io/en/latest/topics/development.html)
 chapter for information and examples of the code style.
-
-License
--------
-By contributing your code, you agree to license your contribution under the
-terms of the project's license.

DCO

@@ -0,0 +1,36 @@
+Developer Certificate of Origin
+Version 1.1
+
+Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
+660 York Street, Suite 102,
+San Francisco, CA 94110 USA
+
+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.