Proposal: require limited subset of RFC3339 for dates

This updates the section on the used date format. 

I've copied a section from the "Federation Service API v2" about RFC 3339 and added that here. I think it is good to explicitly mention the fact that RFC 3339 still allows a lot here, and require a subset of RFC 3339.

Any thoughts?




A side note: As far as I know, on all current aggregates, dates are encoded with RFC3339 and send as XML-RPC `string` type. However, XML-RPC has a native `dateTime.iso8601` type, which is not used. Does anyone know why? Has this to do with library support? The current draft of the API  documents and requires what is currently already done on all aggrates: send dates as RFC3339 in XML-RPC `string`