doc, add example section to README.md bragging about users of unserding

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
Merge branch 'doc/general' into next

* doc/general:
  doc, add example section to README.md bragging about users of unserding
Merge branch 'next'

* next:
  doc, add example section to README.md bragging about users of unserding
  hygiene, mark unsigned operations with U suffix and use restricted pointers
  hygiene, migrate to stdint.h's uint8_t for payload data in private structs
  cosmetics, turn to uint8_t ptrs in ud_pack_cmsg(), ...
  minor, unpack extended sizes in ud_chck_msg()
  minor, allow more than 256U bytes payload by occupying the higher 4bits of the type