[nrf52840] update platform README.md (#2702)
[nrf52840] update platform README.md (#2702)


mszczodrak/openthread,librasungirl/openthread,georgecpr/openthread,erja-gp/openthread,abtink/openthread,LeZhang2016/openthread,jwhui/openthread,LeZhang2016/openthread,lanyuwen/openthread,srickardti/openthread,bukepo/openthread,georgecpr/openthread,abtink/openthread,mszczodrak/openthread,openthread/openthread,srickardti/openthread,lanyuwen/openthread,LeZhang2016/openthread,turon/openthread,openthread/openthread,abtink/openthread,librasungirl/openthread,LeZhang2016/openthread,openthread/openthread,gandreello/openthread,chshu/openthread,openthread/openthread,mszczodrak/openthread,LeZhang2016/openthread,chshu/openthread,chshu/openthread,gandreello/openthread,bukepo/openthread,georgecpr/openthread,mszczodrak/openthread,erja-gp/openthread,turon/openthread,lanyuwen/openthread,gandreello/openthread,turon/openthread,gandreello/openthread,bukepo/openthread,LeZhang2016/openthread,lanyuwen/openthread,erja-gp/openthread,jwhui/openthread,chshu/openthread,erja-gp/openthread,gandreello/openthread,georgecpr/openthread,turon/openthread,srickardti/openthread,chshu/openthread,lanyuwen/openthread,lanyuwen/openthread,jwhui/openthread,gandreello/openthread,abtink/openthread,srickardti/openthread,mszczodrak/openthread,chshu/openthread,librasungirl/openthread,georgecpr/openthread,mszczodrak/openthread,erja-gp/openthread,georgecpr/openthread,bukepo/openthread,jwhui/openthread,librasungirl/openthread,erja-gp/openthread