Add PyPI warning and OS compatibility notes (fixes #179)

[AmanKushwaha-17]

Summary

This PR updates the README.md to address the installation issue described in Issue #179, where users installing the PyPI package encounter:

ModuleNotFoundError: graph_weather.data.nnjai_wrapp

Root Cause

The PyPI package (graph-weather) is out-of-date and still references modules that no longer exist in the GitHub version of the codebase.

Changes Introduced

• Added a clear PyPI warning under the Installation section, explaining the import error and providing a workaround.
• Added a GitHub installation workaround (pip install git+https://github.com/openclimatefix/graph_weather.git) and local editable install instructions.
• Added an Operating System Compatibility section with guidance for Windows users.
  • Clarifies that installation works on Linux/macOS/Windows.
  • Notes that WSL2 may be used optionally if native Windows setup causes issues.
• No code was changed — this PR is documentation-only.

These updates ensure that new users can install the library without hitting the outdated PyPI error, and they improve onboarding clarity.

Closes: #179