[doc] update README.md

Version 1.2.1

Version 1.2.0

Version 1.1.0

- New keyword arguments, `save_intermediate_ext` and `save_intermediate_postprocess`, in the reconstruction functions specifies file extension (image file format; e.g., 'jpg', 'tiff', ...) and post-processing function (e.g., normalization, clipping, ...) for intermediate reconstructed images during iterations, respectively.
- Some utility functions were renamed.

Version 1.0.1

Update README files