Add datafusion design goals / invariants to repository

[alamb]

As part of #55 (review), @jorgecarleitao refers to the following document that tries to summarize a proposed set of DataFusion invariants:

https://docs.google.com/document/d/1Asnz29uUS1t60QNbNBU9SiME274rja-hcDvX_RDraFU/edit

We would send this document around to the mailing lists (dev@arrow.apache.org) (again) if we haven't already and then when sufficient consensus is reached, convert it to markdown and add it to this repo

[jorgecarleitao]

Wow, thanks a lot, @alamb ; excelent idea; I think the document is really old, and we did not have many of the practices and traction we have today

[alamb]

@jorgecarleitao -- if you are interested in this, perhaps I can start the discussion on the mailing list / review the doc. It has been a while :)

[houqp]

+1 on formalizing this doc into datafusion repo!