Merge pull request #6 from getyourguide/contributing-md

Introduce CONTRIBUTING.md

Author: marinofelipe

CONTRIBUTING.md

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+# Contributing
+
+When contributing to this repository, we encourage you to first discuss the change you wish to make via an issue before submitting a change. 
+We're not strict about this, and for small changes, feel free to open a pull request directly.
+
+## Local development
+To run it locally during development:
+
+#### CLI
+run `swift run spmgraph <command> <options>`
+
+#### Xcode (with the debugger attached)
+
+Leverage the [shared spmgraph xcscheme](./.swiftpm/xcode/xcshareddata/xcschemes/spmgraph.xcscheme)
+  - Update the scheme with a custom working directory
+  - Update the arguments with the path to your Package.swift
+  - **Do not commit** such changes
+
+## Pull Request Process
+
+- Aim to keep pull requests scoped to one feature to ease review
+- Add tests to verify that the changes work (if possible)
+- Include or update documentation for new or changed features