Add info about Zone IDs (#730)

Author: QuinnyPig

README.md

@@ -2365,7 +2365,7 @@ This section covers tips and information on achieving [high availability](https:
 
 ### High Availability Gotchas and Limitations
 
--	🔸**AZ naming** differs from one customer account to the next. Your “us-west-1a” is not the same as another customer’s “us-west-1a” — the letters are assigned to physical AZs randomly per account. This can also be a gotcha if you have multiple AWS accounts.
+-	🔸**AZ naming** differs from one customer account to the next. Your “us-west-1a” is not the same as another customer’s “us-west-1a” — the letters are assigned to physical AZs randomly per account. This can also be a gotcha if you have multiple AWS accounts. Note that Zone IDs are consistent between accounts, and can be used to reliably align between AWS accounts.
 -	🔸💸**Cross-AZ traffic** is not free. At large scale, the costs add up to a significant amount of money. If possible, optimize your traffic to stay within the same AZ as much as possible.
 
 Billing and Cost Management