docs(fill): explain how to verify fixture refactors via hasher

[danceratopz]

Add something along the lines of the following which was taken from #1170 (review)

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For future reference, here's how I verified fixtures locally:

# Fill with your branch: with flag 
rm -rf fixtures-1170/; fill --fork Shanghai tests/shanghai/eip3860_initcode/test_initcode.py ----output=fixtures-1170
# Fill with main:
rm -rf fixtures-main/; fill --fork Shanghai tests/shanghai/eip3860_initcode/test_initcode.py ----output=fixtures-main

# Run hasher on both output dirs:
uv run hasher fixtures-1170 > after.txt
uv run hasher fixtures-main > before.txt

#    Diff hasher output:
diff before.txt after.txt