document the idea of payloads

[orangecms]

see also:

• https://doc.coreboot.org/payloads.html
• https://mail.coreboot.org/hyperkitty/list/coreboot@coreboot.org/thread/W4BSV6JJGV3Z3UB3DAH4EWNKMBMWPHGP/
• https://universalscalablefirmware.github.io/documentation/2_universal_payload.html