Create CONTRIBUTING.md

Read this before contributing

CONTRIBUTING.md

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+## Contributing to xNode
+💙Thank you for taking the time to contribute💙
+
+If you haven't already, join our [Discord channel](https://discord.gg/qgPrHv4)!
+
+## Pull Requests
+Try to keep your pull requests relevant, neat, and manageable. If you are adding multiple features, try splitting them into separate commits.
+* Avoid including irellevant whitespace or formatting changes.
+* Comment your code.
+* Spell check your code / comments
+
+## New features
+xNode aims to be simple and extendible, not trying to fix all of Unity's shortcomings.
+
+If your feature aims to cover something not related to editing nodes, it generally won't be accepted. If in doubt, ask on the Discord channel.
+
+## Coding conventions
+Skim through the code and you'll get the hang of it quickly.
+* Methods, Types and properties PascalCase
+* Variables camelCase
+* Public methods XML commented
+* Open braces on same line as condition
+* 4 spaces for indentation.