Add rule 169

[dweber019]

We can't actually check the format but we can use a should rule to provide examples of the format so consumers are supported better.

Was do you think?

We could even check the content of the example filed for the RFC 3339, but we would need to get a good regex or use a library. Should we do this?

[kullmanp]

I like the idea of requiring an example and we could actually lint it (but I would only do it if it is not too hard).

[dweber019]

Thx, let's go with the example first and check linting for example later.