Add DCO and mention DCO in contribution guide

This adds a standard DCO (text taken from
https://developercertificate.org/) and clarifies that all
contributions are subject to this DCO.

This commit also fixes the link to the contribution guide in the
README by changing the extension to lower-case (which is more usual,
anyway).

Author: Max Jonas Werner

CONTRIBUTING.MD renamed to CONTRIBUTING.md

@@ -5,6 +5,10 @@ open source product with a community led by Weaveworks :heart:
 
 We welcome improvements to reporting issues and documentation as well as to code.
 
+## Developer Certificate of Origin
+
+By submitting any contributions to this repository as an individual or on behalf of a corporation, you agree to the [Developer Certificate of Origin](DCO).
+
 ## Understanding how to run development process
 
 The [internal guide](doc/development-process.md) **is a work in progress** but aims to cover all aspects of how to
@@ -26,4 +30,4 @@ In general, we will merge a PR once at least one maintainer has endorsed it. For
 
 Limit the subject to 50 characters and write as the continuation of the sentence "If applied, this commit will ..."
 Explain what and why in the body, if more than a trivial change; wrap it at 72 characters.
-The [following article](https://cbea.ms/git-commit/#seven-rules) has some more helpful advice on documenting your work.
+The [following article](https://cbea.ms/git-commit/#seven-rules) has some more helpful advice on documenting your work.

DCO

@@ -0,0 +1,34 @@
+Developer Certificate of Origin
+Version 1.1
+
+Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
+
+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.