feat: add more instructions

You need to gem install bundle if it isnt installed yet, and recursive gets submodules in cases where init doesnot, I think.

script/bootstrap

@@ -4,9 +4,10 @@ set -e
 export CC=gcc
 
 echo "==> Installing gem dependencies…"
+gem install bundle
 bundle check --path vendor/gems 2>&1 > /dev/null || {
   time bundle install --binstubs bin --path vendor/gems
 }
 
 echo "==> Getting submodule for docs…"
-git submodule update --init
+git submodule update --init --recursive