Add documentation for sub-daily data

[josephguillaume]

Issues with sub-daily data are a recurring issue on the mailing list.
POSIXct indices are not supported but chron indices are. This can be converted easily: index(DATA)<-as.chron(index(DATA))

It is not clear where this documentation should be added or what else needs to be said.
An example would likely be worthwhile.
It is possible that checks for POSIXct indices or automatic conversion to chron may also be worthwhile.

[josephguillaume]

Sub-daily data is often also short data (see #171) where the default objective function with monthly NSE is not appropriate.

[josephguillaume]

Could be simplified if #31 is implemented

[josephguillaume]

Conversion of date indices is included in #186, but it looks like POSIXct is not actually the problem.
Closing this issue in favour of including a couple of common gotchas in #38