[support] Adding an additional table to views

[Yasni]

Hi,

I've created a donation system using CRM and Drupal Commerce.

I have a view for the most recent 10 donations. All the details I need are
being returned from the CRM table.

Now after the site has gone live the client recognises that in this list of
10 recent donation is missing the Country.

That is stored in Drupal commerce as the billing address and not CRM.

What I need is a few really good references for how to add additional tables
to views. And likely a bit of advice on the complexity of doing this.

Yani