Add governance guidelines

GOVERNANCE.md

@@ -0,0 +1,31 @@
+# Governance
+
+The `message_ix` team welcomes contributions of any kind from anyone!
+If you are looking to understand how the team operates and how we decide what to do with your contributions, please read on.
+
+Decisions in the `message_ix` framework are made following good scientific practice, that is, only after careful discussion and review from at least another colleague.
+Depending on the size of the impact the decision will likely have, different scales of decision making are employed.
+For small decisions, the decision may be handled by the [maintainers](#maintainers), while for larger decisions, discussion in the [weekly team meetings](#weekly-team-meetings) may be required.
+
+## Maintainers
+
+At the moment, `message_ix` is maintained by @khaeru and @glatterf42.
+This mostly means that they are most active in responding to issues and discussions and perform the housekeeping work as it arises.
+However, given the size of the `message_ix` framework, it is all but impossible to be truly familiar with every aspect of it.
+For this reason, incoming questions may also be answered by various [specialists within our team](#specialists).
+
+## Specialists
+
+The `message_ix` framework covers many topics already and can be expanded to cover many more and coupled to various other scientific software.
+Examples of this include the modules for water, buildings, material, and transport, or land-use emulators such as [MAgPIE](https://www.pik-potsdam.de/en/institute/departments/activities/land-use-modelling/magpie).
+For all these topics, individual specialists within our team collaborate with one another.
+We encourage everyone to use [trunk-based development](https://www.atlassian.com/continuous-delivery/continuous-integration/trunk-based-development) and [GitHub projects](https://docs.github.com/en/issues/planning-and-tracking-with-projects/learning-about-projects/about-projects) to develop their individual version of our code, while maintaining compatibility with other version.
+We then encourage everyone to frequenly bring their changes to our code back to our `main` development branch. 
+
+## Weekly Team Meetings
+
+Every Thursday, the `message_ix` core developer team meets to discuss current developments.
+This includes discussing proposed changes to the core formulation or new feature ideas in detail.
+
+Once a month, this meeting is extended to invite everyone at IIASA with an interest in `message_ix`.
+These meetings serve the purpose of giving a high-level overview as well as receiving feedback from people who do not join the other meetings. 

README.md

@@ -52,9 +52,13 @@ We aim to create a community that is welcoming to everyone. Please respect our [
 
 We warmly welcome all contributions to our community. Please take a look at our [guidelines for contributing to development](https://docs.messageix.org/en/latest/contributing.html).
 
+## Governance
+
+If you are looking to understand how the `message_ix` team operates and how we decide what to do with your contributions, please read our [governance guidelines](GOVERNANCE.md).
+
 ## Security
 
-We highly value code security. To learn more about our security practice and advice, please visit [our security policy](.github/SECURITY.md).
+We highly value code security. To learn more about our security practice and advice, please visit our [security policy](.github/SECURITY.md).
 
 ## Support