Add contributing doc

CONTRIBUTING.md

@@ -0,0 +1,29 @@
+# Contributing to Simple Icons CDN
+
+## Ways you can help
+
+### Financial contributions
+
+- [GitHub Sponsor](https://github.com/sponsors/LitoMore)
+- [PayPal](https://paypal.me/LitoMore)
+
+## Coding guidelines
+
+### Deno
+
+This project written in Deno. You can get Deno from [deno.com](https://deno.com).
+We usually use the latest version of Deno for development, so please feel free to install the latest version.
+
+### Tests
+
+When adding or changing features please write tests. We're using Deno's built-in [`Deno.test()`](https://docs.deno.com/runtime/fundamentals/testing/) utilities for testing.
+
+You can use `deno test` to run unit tests. And use `deno task test` to test all code formats, linting, types, and unit tests.
+
+### Coverage
+
+We require code coverage to be 100% all the time. You can use `deno task test --html` to generate a coverage report in HTML.
+
+### Development
+
+Use `deno task dev` to start a development server, watching for changes and running on a specific port.

deno.json

@@ -12,6 +12,7 @@
 		"exclude": [
 			".vscode",
 			"coverage",
+			"CONTRIBUTING.md",
 			"README.md"
 		]
 	},