Update invoice-email.md

Hey Guys, 
I have added a couple of notes on some tags that do not render custom CRM data on a normal web form taking payment.

I started an opening paragraph about the 3 cases an invoice email is fired.

I am not sure the best way to present the information on which tags work where.

Maybe You actually do 3 tables here?

First table - invoice: eCommerce
Second table - invoice: Custom web forms
Third table - invoice: Event booking

Because I have foundt hat the custom crm ones as mentioned dont seem to output on a custom web form with payment.
BUT
Shipping attention works, I included it in the form as an input field, set some data and it came through in the invoice.

You got obvious ones like shipping is only relevant to eCommerce and the custom tags as well.

Product description and code change depend on the use case as well.

For eCommerce it is the product description and name and product id.
For an Event it is the event name for both name and description and id is the event id
For a custom webform taking payment the name and description are the web form name (can not be changed front end) and the ID is the web form id.

I really think as a useful reference splitting the single table into three showing what works in what case here is pretty important.
Merge pull request #12 from thenexus00/patch-7

Update invoice-email.md