Update readme to reflect invariant field handling

[SBeairsto]

📚 Update README for Invariant Field Support

Now that invariant field preprocessing is fully supported, the README should be updated to reflect the new functionality.

✅ Key updates to include:

• How to declare invariant fields in the config:
  • Use invariant: true on each ClimateVariable
  • Optionally set downsample: true at the ClimateModel level to coarsen HR invariant fields
• Explanation that invariant fields are not split into train/val/test
  • They are written once to the invariant/ output folder
• Clarify output format:
  • Invariant .pt files are saved as 2D tensors [x, y]
  • Output structure: output_path/invariant/<var>/<model>/<var>.pt
• Brief mention that all fields from WRF constants (e.g., HGT, LANDMASK, LU_INDEX, etc.) are now supported

Updating this will ensure users know how to configure and work with invariant fields correctly.