docs: add contrib guidelines

CONTRIBUTING.md

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+## How you can contribute
+
+Contributions to this project are always welcome. If you spot a bug, or want to request a new extension method, open a new issue
+or submit a pull request.
+
+### Pull request guidelines
+
+This project uses C# 11.0 language features where feasible, and adheres to StyleCop rules with some minor adjustments.
+There is an `.editorconfig` included in this repository. For quick and painless pull requests, ensure that the analyzer does not
+throw warnings.
+
+Please ensure that you follow the [Conventional Commits](https://www.conventionalcommits.org/en/v1.0.0/)
+specification, as the GitHub release for this project is automatically generated from the commit history, and formatted using the
+convetional commits specification.
+
+### Code style
+
+Below are a few pointers to which you may refer, but keep in mind this is not an exhaustive list:
+
+- Use C# 11.0 features where possible
+- Try to ensure code is CLS-compliant. Where this is not possible, decorate methods with `CLSCompliantAttribute` and pass `false`
+- Follow all .NET guidelines and coding conventions.
+  See https://docs.microsoft.com/en-us/dotnet/csharp/fundamentals/coding-style/coding-conventions
+  and https://docs.microsoft.com/en-us/dotnet/standard/design-guidelines/
+- Make full use of XMLDoc and be thorough - but concise - with all documentation
+- Ensure that no line exceeds 130 characters in length
+- Do NOT include file headers in any form
+- Declare `using` directives outside of namespace scope
+- Avoid using exceptions for flow control where possible
+- Use braces, even for single-statement bodies
+- Use implicit type when the type is apparent
+- Use explicit type otherwise
+- Use U.S. English throughout the codebase and documentation
+
+When in doubt, follow .NET guidelines.
+
+### Tests
+
+When introducing a new extension method, you must ensure that you have also defined a unit test that asserts its correct behavior.
+The code style guidelines and code-analysis rules apply to the `X10D.Tests` as much as `X10D`, although documentation may
+be briefer. Refer to existing tests as a guideline.
+
+### Disclaimer
+
+In the event of a code style violation, a pull request may be left open (or closed entirely) without merging. Keep in mind this does
+not mean the theory or implementation of the method is inherently bad or rejected entirely (although if this is the case, it will
+be outlined)