Document date format for property searching

[cfjedimaster]

On https://github.com/heremaps/xyz-documentation/blob/master/docs/api/devguide/propertiessearch.md, document that you should store dates as YYYY-MM-DD if you want to do property based filtering.

[burritojustice]

ISO 8601 makes your life easier in general -- that will be good to add as a tip.

The CLI has an option to generate an ISO-friendly datestring using upload --date