README.md: fix typo
README.md: fix typo

additiona->additional

mdigiorgio/lisa,credp/lisa,joelagnel/lisa,ARM-software/lisa,arnoldlu/lisa,ARM-software/lisa,bjackman/lisa,ARM-software/lisa,credp/lisa,credp/lisa,JaviMerino/lisa,bjackman/lisa,credp/lisa,arnoldlu/lisa,joelagnel/lisa,mdigiorgio/lisa,ARM-software/lisa