[MPEG-OTSPEC] VARC CFF2 hint guidance proposal, and links to others

[Skef Iterum]

Hello -

The last proposal we are submitting prior to the April deadline provides 
guidance on hinting the CFF2-derived components of a composite VARC glyph:

https://github.com/adobe-type-tools/opentype-spec-drafts/blob/main/varchintguide.pdf

This also adds a new optional "CFSH" table for storing PrivateDICT and 
FDSelect structures related to VARC. This new table isn't 100% necessary 
-- one could almost entirely duplicate its function in the CFF2 table 
itself -- but it allows separation of CFF2 component data and VARC 
composite data. Anyway, we can talk about it.

I've also added the earlier documents to our repository:

https://github.com/adobe-type-tools/opentype-spec-drafts/blob/main/newfeatvar_spec.pdf
https://github.com/adobe-type-tools/opentype-spec-drafts/blob/main/negation.pdf
https://github.com/adobe-type-tools/opentype-spec-drafts/blob/main/cff2hintorder.pdf

Only the last has been edited, and only to change "master" to "instance" 
as suggested.

Skef