Document how to use per-recipient substitution data, metadata, etc.

[rdawemsys]

The Transmissions API supports substitution data, metadata and tags are the recipient level, but there's no explanation of how to do this in the documentation. Only this text in https://python-sparkpost.readthedocs.io/en/latest/resources/transmissions.html :

Substitution data can be specified at the template, transmission and recipient levels. The order of precedence is as follows: recipient overrides transmission overrides template.

For reference, substitution data is supported by passing in a dictionary to the send method. E.g.:

response = sp.transmissions.send(
...
  recipients = [
    {
      'address': { 'email': 'recipient@somewhere.example' },
      'substitution_data': { 'name': 'Rachael' }
    }
  ],
...
)

The same may work for metadata and tags, but I have not tested that (yet).

I'm using version 1.3.5.

[jgzamora]

@rdawemsys I'll look into this. It looks like the code passes each entire recipient object without so it should for metadata and tags as well

[jgzamora]

Addressing this in #178