Create CONTRIBUTING.md

Author: stackotter

CONTRIBUTING.md

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+# Contributing
+
+## Environment setup
+
+- Fork and clone SwiftMacroToolkit
+- Open the package in your editor of choice and you're ready to code, have fun!
+
+## Contribution workflow
+
+- Look through the issues on GitHub and choose an issue to work on (or open one if you have an idea)
+- Leave a comment on the issue to let people know that you're working on it
+- Make necessary changes to the codebase
+- Open a PR, making sure to reference the issue that your changes address
+- If a maintainer requests changes, implement the changes
+- A maintainer will merge the changes and the issue can be closed
+
+Thank you for improving SwiftMacroToolkit!
+
+## Before opening a PR
+
+- [ ] Document any new code and update existing documentation as necessary
+- [ ] Ensure that you haven't introduced any new warnings
+- [ ] Check that `swift build` and `swift test` both succeed
+
+Feel free to open a draft PR before you've completed the checklist if you want feedback.
+
+## Package structure
+
+- `Sources/MacroToolkit`: The toolkit itself
+- `Sources/MacroToolkitExample`: Declarations of some example macros which are tested by the tests
+- `Sources/MacroToolkitExamplePlugin`: Implementations for the macros declared by `MacroToolkitExample`
+- `Tests/MacroToolkitTests`: The tests for the toolkit. They simply test that the example macros are
+  working correctly, acting as integration tests for the toolkit. Ideally these will eventually be
+  complimented by more specific unit tests.
+
+## Codestyle
+
+- 4 space tabs
+- Add comments to any code you think would need explaining to other contributors.
+- Document all methods, properties, classes, structs, protocols and enums with documentation comments (if it's trivial,
+  you can just keep the documentation comment short). In Xcode you can press option+cmd+/ when your cursor is on a
+  declaration to autogenerate a template documentation comment (it mostly works).
+- Avoid using shorthand when the alternative is more readable at a glance.