src/Pure/README

updated README;

                        Pure: The Pure Isabelle System


This directory contains the ML source files for Pure Isabelle, which
is the basis for all object-logics.  Building the Isabelle/Pure heap
image in batch mode works as for any other session:

  isabelle build -b Pure

To explore the bootstrap of Pure interactively, the raw ML console can
be used like this:

  isabelle console -l RAW
  use "ROOT.ML";