Document "annotations" YAML option

[alexellis]

Annotations act just like labels, but allow a broader range of values in the value field when using Kubernetes. Kubernetes also uses annotations for other purposes.

Users are not aware that the feature is available - openfaas/faas#803

@ewilde please could you document this new flag in the YAML reference section?

Thanks

[ivanayov]

+1 for this

I noticed that annotations are supported by the CLI, but we don't provide a flag for them.
Should we add the flag to faas deploy? This would allow applying them to functions, deployed from the store, which cannot be done at the moment.

[ivanayov]

Derek add label: area/docs