Create CONTRIBUTING.md

Author: joetannenbaum

.github/CONTRIBUTING.md

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+# Contributing
+
+Thanks for your interest in contributing to Laravel Wayfinder!
+
+```sh
+git clone https://github.com/laravel/wayfinder.git wayfinder
+cd wayfinder
+```
+
+Next, install the dependencies:
+
+```sh
+composer install
+npm install
+```
+
+## Testing
+
+The tests are written using [Vitest](https://vitest.dev/) and can be found in `./tests`. To add a route and/or controller to test against, please reference the `./workbench` directory.