Improve contribution guidelines

Author: sindresorhus

.github/contributing.md

@@ -3,17 +3,22 @@
 ## Submitting a new type
 
 - One type addition per pull request, unless they are connected.
-- Please help review the other open pull requests. If there are no open pull requests, provide some feedback on some of the open issues.
+- **Please help review the other open pull requests.**
+	- If there are no open pull requests, provide some feedback on some of the open issues.
+- [Example of a type contribution.](https://github.com/sindresorhus/type-fest/commit/5374588a88ee643893784f66367bc26b8e6509ec)
 - Create a new file in the `test-d` directory and write at least one type test.
 	- See the other tests for inspiration.
 	- If it makes sense, also write a negative test using [`expectNotAssignable()`](https://github.com/SamVerschueren/tsd#expectnotassignabletexpression-any) or, to test other diagnostics, [`expectError()`](https://github.com/SamVerschueren/tsd#expecterrort--anyexpression-t).
 - Don't use one-character type names like `T` and `U`. Use descriptive names. See the existing types for inspiration.
+- Follow the existing code style, even in documentation code examples.
 - Write a good documentation comment that includes:
 	- Write a short and clear description of what the type does.
+		- The first line should match the description in the readme.
 	- Write about some real-world use-cases where it can be useful. (It can be hard sometimes for users to see where they would use something)
 	- Example code block with a realistic example.
 	- At the bottom, explain how the type works. Some types can be quite advanced and hard to understand. We can use this opportunity to teach users.
 	- If there has been any discussion somewhere about this type, include a link to it. For example, a discussion on the TypeScript issue tracker.
+	- Add relevant `@category` tags. See other types for examples.
 - If you add any internal helper types, they should still be properly documented and tested.
 - Add the type to the readme.
 - Make sure the file in the `source` directory uses a `.d.ts` extension and not `.ts`.