Specify That No UTF-8 BOM Is Allowed

[riggsd]

Byte Order Mark (BOM) is technically allowed in UTF-8 encoding, but neither required nor recommended. Specify that the BOM is illegal in GUANO UTF-8 so that reading implementations don't have to care.

See: https://en.wikipedia.org/wiki/Byte_order_mark#UTF-8

[riggsd]

Or, it sounds like BOM may be required for a UTF-8 plaintext file on Windows... perhaps the spec should simply warn readers that it may be present?