docs: Update System Requirements with GPU VRAM and quantization details

[Roaimkhan]

This PR updates the System Requirements section in README.md to provide more concrete GPU guidelines.

1. Added VRAM ranges for Gemma-2B and Gemma-7B
2. Included examples of popular GPUs (RTX 3060, 4060, 4090, A6000, H100)
3. Clarified full precision vs quantized requirements (Float16/BF16 ≈19 GB, INT4 ≈5 GB)

This makes it easier for new users to quickly determine whether their hardware can run Gemma without searching externally.

[google-cla[bot]]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.