Add HAND and TWI to README feature matrix

[brendancol]

Reason or Problem

hand() (in xrspatial/hand.py) and twi() (in xrspatial/twi.py) are both implemented with multi-backend support, but neither is listed in the README feature matrix. Anyone scanning the README for available operations won't find them.

Proposal

Add rows for HAND and TWI to the Hydrology section of the README feature matrix.

HAND (Height Above Nearest Drainage) backend support:

• NumPy: native (Kahn's BFS kernel)
• Dask+NumPy: native (iterative tile sweep with BoundaryStore)
• CuPy: CPU fallback
• Dask+CuPy: CPU fallback

TWI (Topographic Wetness Index) backend support:

• NumPy: native
• CuPy: native
• Dask+NumPy: native
• Dask+CuPy: CPU fallback

Stakeholders and Impacts

Documentation only. No code changes.

Drawbacks

None.

Alternatives

N/A.