Update Parquet pandas metadata description on how to handle nullable / extension dtypes
From https://github.com/dask/fastparquet/pull/623#discussion_r685904630

The guidelines on how to store column metadata in the pandas metadata in Parquet (https://pandas.pydata.org/docs/development/developer.html#column-metadata) was written before nullable dtypes existed, so based on that it's not unambiguous how those should be handled. 
(and in general, it's also not clear how to handle general extension types, except for the few hardcoded ones (categorical, datetimetz) that already existed at that point)

cc @martindurant 