Explain UTF-8 BOM rule in readme

Most style decisions are explained in the readme, but I couldn't find the reasoning on why a BOM is considered bad. Where I've looked:

* [the patch](https://github.com/airbnb/javascript/commit/d9cb343b518349982f3b094f2ce48eda8347e6c7) that enabled it, found no commit message body.
* the current code of the rules file, https://github.com/airbnb/javascript/blob/8cf2c70a4164ba2dad9a79e7ac9021d32a406487/packages/eslint-config-airbnb-base/rules/style.js#L472-L474
* [the eslint rule page](https://eslint.org/docs/rules/unicode-bom) mentioned in the patch code comment, doesn't claim it's bad.
* searched the readme for "BOM", "unicode" and "byte order"
* searched issue tracker for "BOM", "unicode" and "byte order"

Could someone explain it, or add search keywords to make the explanation easier to find?

Update: Also, is there a recommendation on how to declare the file encoding instead? I searched the readme for "charset", "encod" and "character set" but no matches.

Start with the README and packages/eslint-config-airbnb-base/rules/style.js around the unicode-bom rule, then review the linked commit and ESLint rule page. Determine what rationale and encoding guidance the documentation should provide, and update the README with searchable terminology once the explanation is established.

Written by the indexing model from the issue text.