Contributing.md: Re-work to incorporate the DCO

The Developer's Certificate of Origin was introduced by the Linux kernel
and is where the 'Signed-off-by' stuff came from.

It's a _not_ a CLA, but just asks the developer to confirm they have the
right to submit their code to the repository.

Add a copy of the DCO to the repository and explain about it in
Contributing.md.

Signed-off-by: Andrew Clayton <andrew@digital-domain.net>

Author: ac000

Contributing.md

@@ -2,15 +2,18 @@
 
 When sending code, please either send signed-off patches or a pull request
 with signed-off commits. This means adding a line that says
-"Signed-off-by: Name \<Email\>" at the end of each commit, indicating that
-you wrote the code and/or have the right to contribute it to this project.
-
-E.g
+"Signed-off-by: Name \<Email\>" at the end of each commit. E.g
 
 ```
 Signed-off-by: Andrew Clayton <andrew@digital-domain.net>
 ```
 
+This signifies that you have read/understood/and agreed to the
+[Developer's Certificate of Origin](DCO). Essentially indicating that you
+wrote the code and/or have the right to contribute it to this project.
+
+This is **not** a CLA.
+
 Also, please write good git commit messages. A good commit message looks like
 this:
 
@@ -34,9 +37,12 @@ line. The header line is what is shown by tools like gitk and shortlog, and
 should summarize the change in one readable line of text, independently of
 the longer explanation.
 
-If emailing patches, it is recommended to use git-send-email(1)
+- If emailing patches, it is recommended to use git-send-email(1).
+- If emailing a pull request it is recommended to use git-request-pull(1).
+- Pull requests can be made via GitHub.
 
 Email should be sent to the project maintainer;
+
 ```
 Andrew Clayton <andrew@digital-domain.net>
-```
+```

DCO

@@ -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.