[json-table-schema][s]: add description field as an example of an additional attribute for a field.
[json-table-schema][s]: date-time to datetime as a type.
Merge branch 'master' of github.com:dataprotocols/dataprotocols
[conf][xs]: no longer show git ribbon.
[sdf][m]: add quick start section and move design principles down (copying first paras to top).