Add local environment setup documentation

[ggyuchive]

Currently, syncup's local environment documentation is missing.
It will be great to add order of command with description and prerequisites for convenient contribution.

e.g.

🚀 Local Development Setup

Prerequisites

• Node.js (v18 or higher)
• pnpm (package manager)

  npm install -g pnpm

Installation

1. Clone the repository

   git clone <repository-url>
   cd syncup

2. Install dependencies

   pnpm install

Running the Application

Frontend only:

pnpm frontend dev

Backend only:

pnpm backend start:dev