Claim sign-off is required

Signed-off-by: Chenxiong Qi <[email protected]>

Author: tkdchen

Diff for: README.rst

@@ -35,6 +35,56 @@ pull request, please make sure your code passes checks. Run
 
   tox
 
+Sign-off commit
+---------------
+
+Every commit must be signed off with your name and email address. This can be
+done by specifying option ``-s`` to ``git commit``, for example::
+
+   git commit -s -m "commit message"
+
+The sign-off means you have read and agree to `Developer Certificate of Origin`_.
+Nitrate uses version 1.1::
+
+   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.
+
+.. _Developer Certificate of Origin: https://developercertificate.org/
+
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