Change device reader from .harp to .yml in readme

[ablot]

Updated device reader examples to use 'device.yml' instead of 'device.harp' to match the file description above.

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[bruno-f-cruz]

The current documentation is consistent with the function signature and documentation:

harp-python/harp/reader.py

Lines 242 to 244 in 3d97d0b

	device
	A path to the device schema, dataset folder, or parsed device schema object
	describing the device.

While you can pass a device.yml, the "canonical" use is actually to pass a full folder since the reader can infer the path to all the files. We are trying to expand the current API via #26 to make this a bit more explicit by using different factories for the reader.

I will close this PR for now, but feel free to open an issue if you want to continue the discussion!

Thanks :)