1. Host 机中的learning目录（即 Vagrantfile所在同一目录下新建learning目录）
2. 虚拟机中的/home/vagrant/learning

# Share an additional folder to the guest VM. The first argument is
# the path on the host to the actual folder. The second argument is
# the path on the guest to mount the folder. And the optional third
# argument is a set of non-required options.

config.vm.synced_folder "learning/", "/home/vagrant/learning"