Create a stateful counter button with unit tests and update README

[achan214]

Description

Incremental testing done by developers themselves can prevent big bugs later on that have to be fixed. We would like you guys to start implementing testing before pushing your code! This is an example issue for you guys to practice testing.

• Create a simple counter button on a new page page in /src/app/[username]/page.tsx
• Write a unit test to validate that the button functions properly

Acceptance Criteria

• A new frontend route is created and contains a button that increments a counter on click

Steps:

• Create a new branch to work on in the form username-intro(e.g. achan214-intro)
• Create a new route as /src/app/[username]/page.tsx
• Create the new button on this new page using the button component from shadcn. The button text should be the counter, which you can start at 0
• Create a test file page.test.tsx in /test/app/[username]. Use jest to test that the button increments on click
• Make a PR, we will not merge your code but we want to see that you have completed the example issue