Add PyPI installation instructions to README

[Copilot]

Summary

Added PyPI installation instructions to the README's Installation section. The package is available on PyPI as mycelium-ai but the README only mentioned source installation.

Changes

• Added pip install mycelium-ai as the primary installation method
• Reorganized the Installation section to show PyPI installation first
• Kept source installation as an alternative for developers

Before

The README only showed source installation:

# Clone and install
git clone https://github.com/marceljungle/mycelium.git
cd mycelium
pip install -e .

After

Now includes PyPI installation as the primary method:

# Install from PyPI
pip install mycelium-ai

Or install from source:

# Clone and install
git clone https://github.com/marceljungle/mycelium.git
cd mycelium
pip install -e .

This makes it easier for users who just want to use Mycelium without needing to clone the repository.

Original prompt

Introduce in the readme that the package can also be installed through pip install since it is uploaded to pypi. Keep it simple, go straight to the point.

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