Add README and organize checkpoint files

README.md

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+# MethaneFuse Checkpoints
+
+This repository hosts model checkpoints for MethaneFuse, a two-stage framework for multi-sensor satellite methane plume detection.
+
+## Checkpoints
+
+| File | Description |
+|---|---|
+| `stage1/methanefuse_stage1_pretrained.pt` | Stage 1 sensor-native pretrained encoder |
+| `classification/methanefuse_cls_120m.pt` | Stage 2 classifier for 120 m query footprint |
+| `classification/methanefuse_cls_360m.pt` | Stage 2 classifier for 360 m query footprint |
+| `classification/methanefuse_cls_480m.pt` | Stage 2 classifier for 480 m query footprint |
+| `classification/methanefuse_cls_960m.pt` | Stage 2 classifier for 960 m query footprint |
+
+## Code
+
+Code and usage instructions are available at:
+
+https://github.com/yuyao-wang/MethaneFuse