Updated github

As promised I have transcribed the updates from the spreadsheet to the
respec document on Github.

more work to do to clean it up w.r.t. groupings

So far:
1) fixed up some heading level inconsistencies
2) added requirements groupings headings and  group definition text as per
spreadsheet
3) commented out rejected requirements
4) Updated heading and/or text where "final" text supplied

To note - largely kept original ordering of group headings (spreadsheet was
alphabetical)
TODO - verify this makes sense

also numbering is automated, IMHO we should finalise all text and groupings
and then add numbers based on groups,

Lots of requirements still need to be shuffled into appropriate groups.
I'll attack this next after i get feedback on numbering strategy,

Rob Atkinson