Tweak readme

Author: pgleeson

examples/README.md

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 ## Some simple examples of the use of Modelspec
 
 See also packages [NeuroMLlite](https://github.com/NeuroML/NeuroMLlite) & [MDF](https://github.com/ModECI/MDF) for usage of Modelspec.
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+## A simple definition of the structure of a Document in modelspec
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+A Python file defining the structure of the Document, as well as saving an instance: [document.py](document.py).