README: Add recursive to git submodule update command
README: Add recursive to git submodule update command

Recursive update is required for nested submodule

Signed-off-by: Tzach Livyatan <434a324262fc65c9776045522d30d755c828b1aa@cloudius-systems.com>

guiquanz/scylla,wildinto/scylla,asias/scylla,glommer/scylla,shaunstanislaus/scylla,glommer/scylla,shaunstanislaus/scylla,duarten/scylla,capturePointer/scylla,phonkee/scylla,rentongzhang/scylla,raphaelsc/scylla,dwdm/scylla,eklitzke/scylla,scylladb/scylla,victorbriz/scylla,gwicke/scylla,kangkot/scylla,wildinto/scylla,avikivity/scylla,scylladb/scylla,respu/scylla,respu/scylla,duarten/scylla,rluta/scylla,senseb/scylla,aruanruan/scylla,aruanruan/scylla,scylladb/scylla,capturePointer/scylla,rentongzhang/scylla,stamhe/scylla,scylladb/scylla,wildinto/scylla,raphaelsc/scylla,bowlofstew/scylla,eklitzke/scylla,guiquanz/scylla,bowlofstew/scylla,respu/scylla,aruanruan/scylla,kangkot/scylla,guiquanz/scylla,asias/scylla,glommer/scylla,victorbriz/scylla,tempbottle/scylla,capturePointer/scylla,acbellini/scylla,shaunstanislaus/scylla,victorbriz/scylla,phonkee/scylla,senseb/scylla,tempbottle/scylla,justintung/scylla,linearregression/scylla,avikivity/scylla,dwdm/scylla,gwicke/scylla,gwicke/scylla,phonkee/scylla,asias/scylla,senseb/scylla,avikivity/scylla,justintung/scylla,linearregression/scylla,kjniemi/scylla,stamhe/scylla,rluta/scylla,rluta/scylla,raphaelsc/scylla,duarten/scylla,eklitzke/scylla,linearregression/scylla,acbellini/scylla,dwdm/scylla,kjniemi/scylla,acbellini/scylla,kjniemi/scylla,kangkot/scylla,bowlofstew/scylla,rentongzhang/scylla,justintung/scylla,stamhe/scylla,tempbottle/scylla