Update nRF52840 platform's README.md (#1461)
Update nRF52840 platform's README.md (#1461)


pvanhorn/openthread,gandreello/openthread,aeliot/openthread,mszczodrak/openthread,pvanhorn/openthread,JakubBrachTieto/openthread,xiaom-GitHub/openthread,openthread/openthread,LeZhang2016/openthread,chshu/openthread,erja-gp/openthread,aeliot/openthread,nibanks/openthread,librasungirl/openthread,jwhui/openthread,gandreello/openthread,vaas-krish/openthread,georgecpr/openthread,gandreello/openthread,vaas-krish/openthread,librasungirl/openthread,openthread/openthread,srickardti/openthread,mszczodrak/openthread,abtink/openthread,nibanks/openthread,mszczodrak/openthread,jjlee9/openthread,JakubBrachTieto/openthread,turon/openthread,lanyuwen/openthread,jjlee9/openthread,fbsder/openthread,erja-gp/openthread,bukepo/openthread,abtink/openthread,pvanhorn/openthread,srickardti/openthread,xiaom-GitHub/openthread,xiaom-GitHub/openthread,bukepo/openthread,vaas-krish/openthread,aeliot/openthread,LeZhang2016/openthread,LeZhang2016/openthread,lanyuwen/openthread,nibanks/openthread,jjlee9/openthread,jjlee9/openthread,vaas-krish/openthread,aeliot/openthread,mszczodrak/openthread,erja-gp/openthread,chshu/openthread,fbsder/openthread,jjlee9/openthread,georgecpr/openthread,georgecpr/openthread,openthread/openthread,librasungirl/openthread,fbsder/openthread,xiaom-GitHub/openthread,lanyuwen/openthread,xiaom-GitHub/openthread,mszczodrak/openthread,georgecpr/openthread,jjlee9/openthread,fbsder/openthread,librasungirl/openthread,openthread/openthread,lanyuwen/openthread,JakubBrachTieto/openthread,lanyuwen/openthread,nibanks/openthread,gandreello/openthread,LeZhang2016/openthread,jwhui/openthread,abtink/openthread,erja-gp/openthread,pvanhorn/openthread,aeliot/openthread,JakubBrachTieto/openthread,fbsder/openthread,LeZhang2016/openthread,gandreello/openthread,LeZhang2016/openthread,vaas-krish/openthread,bukepo/openthread,erja-gp/openthread,jwhui/openthread,xiaom-GitHub/openthread,pvanhorn/openthread,pvanhorn/openthread,aeliot/openthread,nibanks/openthread,fbsder/openthread,jwhui/openthread,chshu/openthread,srickardti/openthread,turon/openthread,gandreello/openthread,georgecpr/openthread,lanyuwen/openthread,chshu/openthread,nibanks/openthread,JakubBrachTieto/openthread,chshu/openthread,JakubBrachTieto/openthread,bukepo/openthread,vaas-krish/openthread,abtink/openthread,turon/openthread,turon/openthread,chshu/openthread,georgecpr/openthread,srickardti/openthread,erja-gp/openthread,mszczodrak/openthread