Readme: add links and references to OME-NGFF 0.4.1 specification

[sbesson]

Following the release of bioformats2raw 0.5.0 and the announcement of the OME-NGFF 0.4.1 specification, this reviews the README to:

• include links to the OME-NGFF specification which defines the layout of Zarr datasets generated by this utility
• update the usage paragraph to promote the default command and document the --resolutions and --target-min-size advanced options separately
• add a first paragraph about version changes in 0.5.0