Add contribution guidelines

CONTRIBUTING.md

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+# Contribution Guidelines
+
+- If you think you've found a security vulnerability, please email
+  delfin@avery.cafe
+
+## Bug Reports and Feature Requests
+
+You can report bugs or file feature requests on the
+[issue tracker](https://codeberg.org/avery42/delfin).
+
+- Before creating an issue, do a quick search to check if the issue has already
+  been reported.
+- When creating an issue,
+  please choose the appropriate template, and follow it to the best of your
+  ability.
+  - Following the templates helps to ensure that you include the relevant
+    information in your issue, and reduces back and forth.
+
+## Development
+
+See [Development Setup](https://codeberg.org/avery42/delfin#development-setup).
+
+Before working on a change or new feature, please open an issue for discussion.
+At this stage, Delfin aims to provide a cohesive UI for the most important
+features. Before implementation, there should be a discussion about how a new
+feature will fit into the app, along with whether the feature even belongs in
+the first place.
+
+## Translations
+
+Contributions to translations are welcome on
+[Weblate](https://translate.codeberg.org/projects/delfin/).
+
+Delfin's translations are using the
+[Fluent localization system](https://projectfluent.org/). A syntax guide is
+availble [here](https://projectfluent.org/fluent/guide/).
+
+<a href="https://translate.codeberg.org/engage/delfin/">
+    <img src="https://translate.codeberg.org/widget/delfin/open-graph.png" alt="Translation status" width="400px" />
+</a>
+
+<br />
+<a href="https://translate.codeberg.org/engage/delfin/">
+    <img src="https://translate.codeberg.org/widget/delfin/multi-auto.svg" alt="Translation status" />
+</a>