Update documenation for retrieving a list of all templates (dynamic & legacy)

[thinkingserious]

Issue Summary

Please update the USAGE.md and corresponding \example with the call to list all templates.

This is the API call required: GET https://api.sendgrid.com/v3/templates?generations=legacy,dynamic

Acceptance Criteria

• The Usage.md file demonstrates how to use this API call to retrieve all templates: GET https://api.sendgrid.com/v3/templates?generations=legacy,dynamic
• The corresponding /example file demonstrates how to use this API call to retrieve all templates: GET https://api.sendgrid.com/v3/templates?generations=legacy,dynamic

[thinkingserious]

Specifically these locations:
https://github.com/sendgrid/sendgrid-csharp/blob/master/USAGE.md#get-templates
https://github.com/sendgrid/sendgrid-csharp/tree/master/examples

Thanks!

[tony-ho]

@thinkingserious I'll take this one.