Document how to append to the grammar

[jokull]

If there is a way to augment the grammar file for specific project contexts, that should be documented.

[sveinbjornt]

Agreed, we need much more and better documentation of the grammar. Meanwhile, you might want to take a look at Greynir's query handling modules, which extend the grammar to handle a wide range of natural-language queries. They're the ones used by Embla.

https://github.com/mideind/Greynir/blob/master/query.py
https://github.com/mideind/Greynir/tree/master/queries