Expand README with running PySE examples #153

The README should describe how to run PySE:

1. From the command line - hatch run pyse [--options] - in combination with a config.toml, so some example config.toml will need to be provided in order to see how command line overrides config.toml settings.
2. From an iPython shell. It should include the on-the-fly creation of Conf object, which should again override config.toml. The use of the beam 3-tuple should also be demonstrated.
3. For vectorized source processing, i.e. with conf.image.vectorized==True and conf.image.deblend_nthresh==0 show PySE's speed (using %timeit), e.g. on an artificial 4K *4K image with over 100,000 sources, from an iPython shell. Show that a Pandas df is returned and the descriptions of its columns - not just the column headers - can be shown using describe_dataframe_columns.