Create CONTRIBUTING.md

Author: jreina

CONTRIBUTING.md

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+# Contributing to ShittyLINQ
+
+1. Read the [Code of Conduct](./CODE_OF_CONDUCT.md)
+1. If adding a new method, follow the [new method instructions](#adding-a-new-method)
+
+## Adding a new method
+1. If an issue exists for this method, add a comment to let everyone know that you are implementing it. This helps us avoid duplicates.
+1. Fork the repo if you haven't done so already.
+1. Create a branch from an up-to-date `develop` branch.
+1. Add a file at `./ShittyLINQ/<method name>.cs`.
+1. Add the new `public static` method in a `public static partial` class called `Extensions`, which must be in the `ShittyLINQ` namespace.
+1. Implement the method.
+1. Document the method using standard XML documentation.
+1. [Add tests](#adding-a-set-of-tests).
+1. Push branch to your fork.
+1. Create a pull request against the `develop` branch of the main repo.
+
+## Adding a set of tests
+1. Add a file at `./ShittyLinqTests/<method name>Tests.cs`.
+1. Inside of the `ShittyTests` namespace, add your test methods.
+1. Add tests for success conditions.
+1. Add tests for failure conditions according to the MSDN documents for the method you are adding. Ensure the proper exceptions are thrown.