[cosi] Add more discussion topics

[nikkolasg]

This is a tentative PR to bring up all the points that are currently discussed in the CFRG mailing list and curves@moderncrypto mailing list. It fixes issue https://github.com/dedis/doc/issues/2 .

[nikkolasg]

I haven't out yet figured out what would be the best way to emphasize on these topic discussions in the draft itself while keeping the draft somewhat neat for it to still be presentable as an IETF draft...