Add CONTRIBUTING.md and require DCO

Co-authored-by: Filipe Coelho <falktx@falktx.com>
Co-authored-by: Sol Fisher Romanoff <sol@solfisher.com>

.github/pull_request_template.md

@@ -15,7 +15,7 @@ Resolves: *(direct link to the issue)*
 an asterisk (*) after the entry indicates required
 -->
 
-- [ ] The code in this pull request is licensed under "GPLv2 or any later version"\*
+- [ ] I have signed off my commits using `-s` or `Signed-off-by`\* (See: [Contributing § DCO](https://github.com/tenacityteam/tenacity/blob/master/CONTRIBUTING.md#developer-certificate-of-origin))
 - [ ] I made sure the code compiles on my machine
 - [ ] I made sure there are no unnecessary changes in the code\*
 - [ ] I made sure the title of the PR reflects the core meaning of the issue you are solving\*

CONTRIBUTING.md

@@ -0,0 +1,114 @@
+# Contributing to Tenacity
+
+Thanks for showing your interest to contribute, your contribution is very
+valuable to us as people like **you** help us build Tenacity. Some guidelines
+have been put in place in an effort to keep the codebase clean.
+
+## Reporting bugs and feature requests
+
+A great place to ask questions about using Tenacity, report bugs and discuss
+feature requests is the [tenacity-discuss mailing list](https://lists.sr.ht/~tenacity/tenacity-discuss).
+Please search through the archives, as your question may have been asked before.
+We're also available on IRC at #tenacity on Libera.Chat [(web)](https://web.libera.chat/gamja/?channels=tenacity)
+if you'd like a more real-time discussion.
+
+The [Issues tab](https://github.com/tenacityteam/tenacity/issues)
+on GitHub is restricted to **bugs and feature requests**
+only. Please only submit a bug report if you are sure it is valid.
+
+## Contributing
+
+Contributing code to Tenacity is done either via sourcehut or GitHub. Tenacity
+requires you to Sign-off your commits, which indicates you agree to the
+[Developer Certificate of Origin](#developer-certificate-of-origin). Details below.
+
+_Note: you do not need to open a GitHub issue for every matching contribution,
+only for those which need further looking into, and only when asked to._
+
+When contributing, please adhere to the following guidelines:
+
+1. Follow proper code formatting guidelines e.g. If the file uses spaces, do not
+   change them to tabs.
+2. Do not change any variable names unless necessary.
+3. Make concise and accurate commit messages. A commit message should be
+   limited to 50 characters and its description limited to 72 characters
+   per line, and the message should complete this sentence:
+
+    > This commit will `Add support for protocol v3`
+
+    If you need to add any additional context, do so in the commit description.
+
+### Contributing through sourcehut
+
+SourceHut operates on an email-driven workflow, and uses [`git send-email`](https://git-send-email.io) for patch submission. Please send your
+patches to
+[`~tenacity/tenacity-dev@lists.sr.ht`](mailto:~tenacity/tenacity-dev@lists.sr.ht).
+When revising a patch, please use `git commit --amend` and add the `-v2` (increment every revision)
+flag.
+
+### Contributing through GitHub
+
+To contribute code using GitHub, first
+[fork this repository](https://github.com/tenacityteam/tenacity)
+and make your changes. Please use `git commit --amend` and
+`git push -f` for minor changes (only **your** commits).
+See [git-rebase.io](https://git-rebase.io) for more details.
+
+## Developer Certificate of Origin
+
+Tenacity is an open source project licensed under the GNU General Public
+license, version 2 only (see [`LICENSE`](LICENSE)).
+
+We respect intellectual property rights, and we'd like to make sure that all
+contributions are properly attributed. As such, we use the simple and clear
+Developer Certificate of Origin (DCO).
+
+The DCO is a declaration attached to every contribution made by every
+contributor. All the developer has to do is include a `Signed-off-by` statement,
+thereby agreeing to the DCO, provided below or on
+[developercertificate.org](https://developercertificate.org):
+
+```
+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.
+```
+
+Each commit must include a DCO in its description, which looks like this:
+
+```
+Signed-off-by: Jane Doe <j.doe@email.email>
+```
+
+You may type this line manually or, if using the command line, simply append
+`-s` to your commit.
+
+We ask that all contributors use their real email address, which can be replied
+to.