docs: improve contributing guide (#461)

The aim of this change is to make contributions clearer to approach.

CONTRIBUTING.md

@@ -1,9 +1,29 @@
 # Contributing
 
+## Ways to Contribute
+
+- Report bugs
+- Implement performance improvements
+- Refine documentation
+- Suggest new features and design improvements
+- Link to your SaaSKit-related project
+
+## Creating an Issue
+
+- Make sure that an issue doesn't already exist (avoid duplicates)
+- Feel free to ask questions in the
+  [#saaskit Discord channel](https://discord.com/channels/684898665143206084/712010403302866974)
+
 ## Submitting a Pull Request
 
-When submitting a pull request, please follow the
-[Deno Style Guide](https://deno.land/manual/references/contributing/style_guide).
+- Follow the
+  [Deno Style Guide](https://deno.land/manual/references/contributing/style_guide)
+- A pull request must address a given issue. If an issue does not exist, create
+  one
+- Tests must be provided for new functionality
+
+## Design Rules
 
-Before submitting a pull request, please run `deno task ok` and ensure all
-checks pass. This checks formatting, linting, types and runs tests.
+Follow
+[CocoaPods design rules](https://github.com/CocoaPods/CocoaPods/wiki/Communication-&-Design-Rules#design-rules)
+which apply to this project.