Add documentation on how to add validation for custom types

[jd-lara]

It isn't clear currently what's the process to avoid the warning

┌ Warning: struct MyCustomDevice does not exist in validation configuration file, validation skipped
└ @ InfrastructureSystems ~/.julia/dev/InfrastructureSystems/src/validation.jl:42

[daniel-thom]

This means that the descriptor file does not contain the struct being used. We have an oversight in this process. A user might want validation of a struct without doing auto-generation. Is that what is happening here?

It might be better to make this warning message a debug message.

[jd-lara]

we currently throw a warning when there is no valid entry for a custom struct validation. We want to add validation for the DynamicInverter and also make sure that the validations on the field values of components are conducted.

[daniel-thom]

My earlier statement was incorrect. You can define a validation descriptor for a struct that is not auto-generated by adding an entry here: https://github.com/NREL-SIIP/PowerSystems.jl/blob/master/src/descriptors/power_system_structs.json#L9581.

There are a few options:

1. Add the descriptor.
2. Implement a method for the function IS.validate_struct(ist::InfrastructureSystemsType). (validate based on fields in struct)
3. Implement a method for the function PSY.validate_struct(sys::System, value::PowerSystemType). (validate based on system)

[kdayday]

@jd-lara another stale issue -- we currently have very sparse documentation for validation, but this doesn't seem to be a documentation problem on its own. I'm removing the label.