Update Doc on structure of training data

[nathanfletcher]

Update modeling/dataset/ReadMe.md with the following:

• Current source of training data
• Where training data will be coming from (web url/ sheet)
• How it will be reformatted back to the current format
• Any changes that will be made to the process

[lacabra]

And sync with the content at https://unicef.github.io/kindly/ml-model/training-data