Add contribution guide here, too

Related to
briar-gtk!96
and
briar-gtk#88.

CONTRIBUTING.md

+## Contributing to briar_wrapper
+
+Thank you for looking in this file!
+
+When contributing to the development of briar_wrapper, please first discuss the change you wish to make via issue, email, or any other method with the maintainers before making a change.
+
+If you have any questions regarding the use or development of briar_wrapper,
+want to discuss design or simply hang out, please join us in [Briar's Mattermost chat](https://chat.briarproject.org/), [#briar on freenode.net](irc://freenode.net/#briar) or [the bridge to the Matrix network](https://matrix.to/#/#freenode_#briar:matrix.org).
+
+Please note we have a [code of conduct](/code-of-conduct.md), please follow it in all your interactions with the project.
+
+## Source repository
+
+briar_wrapper's main source repository is at [code.briarproject.org/briar/python-briar-wrapper](https://code.briarproject.org/briar/python-briar-wrapper).
+
+Development happens in the _main_ branch.
+
+If you need to publish a branch, feel free to do it at any
+publically-accessible Git hosting service, although code.briarproject.org
+makes things easier for the maintainers.
+
+## Style
+
+We use pycodestyle and pylint for code formatting and we enforce it on the GitLab CI server.
+
+## Running the test suite
+
+You can run the tests with the script [`tools/run-tests.sh`](tools/run-tests.sh).
+
+# Issues, issues and more issues!
+
+There are many ways you can contribute to briar_wrapper, and all of them involve creating issues
+in [briar_wrapper's issue tracker](https://code.briarproject.org/briar/python-briar-wrapper/issues). This is the entry point for your contribution.
+
+To create an effective and high quality ticket, try to put the following information on your
+ticket:
+
+ 1. A detailed description of the issue or feature request
+     - For issues, please note your version and add the necessary steps to reproduce the issue.
+     - For feature requests, add a detailed description of your proposal.
+ 2. A checklist of Development tasks
+ 3. A checklist of Design tasks
+ 4. A checklist of QA tasks
+
+Don't mind if you have nothing to say about point 2), 3) or 4).
+
+## Issue template
+```
+[Title of the issue or feature request]
+
+Detailed description of the issue. Put as much information as you can, potentially
+with images showing the issue or mockups of the proposed feature.
+
+If it's an issue, add the version and steps to reproduce like this:
+
+Steps to reproduce:
+
+1. Start up briar_wrapper by logging in
+2. Do an Action
+3. ...
+
+briar_wrapper version: 0.2.0-beta1
+
+## Design Tasks
+
+* [ ]  design tasks
+
+## Development Tasks
+
+* [ ]  development tasks
+
+## QA Tasks
+
+* [ ]  qa (quality assurance) tasks
+```
+
+## Merge Request Process
+
+1. Ensure your code compiles.
+2. Ensure the test suit passes. Run `tools/run-tests.sh`.
+3. If you're adding new API, it must be properly documented.
+4. The commit message has to be formatted as follows:
+   ```
+   <summary>
+
+   A paragraph explaining the problem and its context.
+
+   Another one explaining how you solved that.
+
+   Fixes <link to the bug ticket>
+   ```
+5. You may merge the merge request once you have the sign-off of the maintainers, or if you
+   do not have permission to do that, you may request the reviewer to merge it for you.
+
+## Code of Conduct
+
+We follow the [Briar Code of Conduct](/code-of-conduct.md).

code-of-conduct.md

+# Briar Code of Conduct
+
+Thank you for being a part of the Briar project. We value your participation and want everyone to have an enjoyable and fulfilling experience. Accordingly, all participants are expected to follow this Code of Conduct, and to show respect, understanding, and consideration to one another. Thank you for helping make this a welcoming, friendly community for everyone.
+
+Please see [briarproject.org/code-of-conduct](https://briarproject.org/code-of-conduct).