[nrf52840] update README.md after SDK release (#3759)
[nrf52840] update README.md after SDK release (#3759)


jwhui/openthread,bukepo/openthread,jwhui/openthread,openthread/openthread,chshu/openthread,bukepo/openthread,srickardti/openthread,librasungirl/openthread,srickardti/openthread,chshu/openthread,srickardti/openthread,lanyuwen/openthread,chshu/openthread,openthread/openthread,jwhui/openthread,openthread/openthread,abtink/openthread,abtink/openthread,chshu/openthread,librasungirl/openthread,turon/openthread,lanyuwen/openthread,chshu/openthread,lanyuwen/openthread,abtink/openthread,srickardti/openthread,lanyuwen/openthread,bukepo/openthread,turon/openthread,openthread/openthread,chshu/openthread,bukepo/openthread,jwhui/openthread,abtink/openthread,lanyuwen/openthread,librasungirl/openthread,librasungirl/openthread,lanyuwen/openthread,turon/openthread,turon/openthread