need a document to describe the flood products

[cirrusasf]

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We change the no-data value for flood_map.tif and water_map.tif, as well as flood_mask.tif, but we do not give this information anywhere in the package. Users will get confused when they get the new release.

Describe the solution you'd like
I suggest we include a README.md for the product.
In the file, we describe the the no-data value for these files.