Add contribution guidelines

Contributing.md

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+# Contributing
+
+DiscordChatExporter accepts contributions in the form of issues and pull requests.
+
+## Creating issues
+
+If you have a feature suggestion or want to report a bug, you are welcome to create an issue. When doing so, please follow these guidelines:
+
+- Avoid creating an issue if a similar one already exists. Look through existing open and closed issues first.
+- Keep your issue focused on one specific problem. If you have multiple suggestions or bug reports, please create separate issues for them.
+- Provide a descriptive title for your issue. Don't use generic titles like "A couple suggestions" or "Not working".
+- Provide more context in the body of the issue. If relevant, attach screenshots or screen recordings.
+- Remain civil and respectful when participating in discussions.
+
+## Creating pull requests
+
+If you want to contribute code to the project, you can create a pull request. When doing so, please follow these guidelines:
+
+- Make sure that there is an existing issue that describes the problem solved by your pull request. This can be ignored for trivial changes.
+- Try to keep the pull request focused and as small as possible. If you want to contribute multiple unrelated changes, please create separate pull requests for them.
+- Follow the coding style and conventions already established by the project. When in doubt which style should be used in a particular case, ask in comments to your pull request.
+- If you want to start a discussion regarding a specific part of the change, add review comments to your own code. This can be used to highlight something important or to seek input from others.
+
+## Building the project locally
+
+Prerequisites:
+
+- [.NET 5.0 SDK](https://dotnet.microsoft.com/download/dotnet/5.0)
+- [.NET 3.1 SDK](https://dotnet.microsoft.com/download/dotnet/3.1) (temporarily as the app still targets .NET 3.1, but requires .NET 5.0 to build)
+- _(Optional)_ C#/.NET IDE, such as [JetBrains Rider](https://www.jetbrains.com/rider), [VS Code](https://code.visualstudio.com/docs/languages/csharp), or [Visual Studio](https://visualstudio.microsoft.com).
+
+To build the entire solution run the following command in the root of the repository:
+
+```sh
+> dotnet build
+```
+
+This will generate runtime artifacts for each project:
+
+```plaintext
+./DiscordChatExporter.Gui/bin/[Debug|Release]/[runtime]/*
+./DiscordChatExporter.Cli/bin/[Debug|Release]/[runtime]/*
+```
+
+You can also build and run a specific project directly.
+To do that, navigate to its directory and use `dotnet run`:
+
+```sh
+> cd DiscordChatExporter.Gui
+> dotnet run
+```