Add README.md related to run playwright tests locally (#722)

frontend/tests/README.md

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+# Running Tests Locally
+
+This guide will show you how to run tests locally for a frontend application using `yarn start` and `npx playwright test`. 
+
+## Prerequisites
+
+Before getting started, you should have the following installed on your computer:
+
+- Node.js
+- Yarn
+
+## Step 1: Clone the repository
+
+Begin by cloning the repository to your local machine.
+
+`git clone git@github.com:webrecorder/browsertrix-cloud.git`
+
+## Step 2: Change directory to frontend
+
+Navigate to the frontend directory:
+
+`cd frontend`
+
+## Step 3: Install dependencies
+
+Install the project's dependencies:
+
+`yarn install`
+
+## Step 4: Add `.env`
+
+Add a `.env` file by copying the sample:
+
+`cp sample.env.local .env`
+
+## Step 5: Start the application
+
+Start the application:
+
+`yarn start`
+
+This will open the application in your browser at `http://localhost:5173`.
+
+## Step 5: Open a new terminal tab
+
+Open a new terminal tab so that the first one continues to run the application.
+
+## Step 6: Run tests
+
+Run the tests in the new terminal tab using `npx playwright test`.
+
+This will run the tests and output the results in the terminal.