Document mirroring rules

[caugner]

What type of issue is this?

Other

What is the issue?

BCD supports a "mirror" support value that causes data to be intelligently mirrored from an upstream browser at build time, but this isn't documented yet.

What behavior were you expecting?

The mirroring should be documented, explaining how the mirroring works, and listing the rules that determine if and how data is mirrored.

What version(s) of BCD is the issue present in?

• The current BCD release
• The current version of the main branch

Do you have anything more you want to share?

Action item from the BCD project meeting on 2024-11-19.