Create CONTRIBUTING

.github/CONTRIBUTING

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+# Contributing to the Open IMO CII Calculator
+
+Thanks for your interest in the project!
+
+## Testing
+
+There are some unit tests in the `EtiveMor.OpenImoCiiCalculator.Core.Tests` directory. The app uses MSTest for all unit testing. 
+
+## Submitting Changes
+
+When making changes, please send a pull request to Open-IMO-CII-Calculator with a clear list of what changes have been made. To understand more about pull requests, please see Github's [Pull Request help page](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/getting-started/best-practices-for-pull-requests).
+
+Pull Requests should contain one feature/change per commit (atomic commits). Pull request messages should be clear and to the point.
+
+Please be explicit where your contribution has used generative AI &/or LLMs to create code (including tools like github copilot), and describe which model was used. 
+
+## Coding Conventions
+
+- Prefer readability over performance
+- Generally follow [dotnet coding conventions](https://learn.microsoft.com/en-us/dotnet/csharp/fundamentals/coding-style/coding-conventions)
+- Methods and classes should be commented with `/// <summary>` style comments describing the method's purpose
+- If a method implements a regulatory requirement, for example `IMO: MEPC.354(78)`, please include a reference to the exact regulation name in the comments