Update README.md

[dotku]

I found that Node 12 (LTS) through error, then the installation will be failed :(

npm install

> [email protected] install /Users/wlin/dev/authentication_course/node_modules/fsevents
> node install

node-pre-gyp WARN Using request for node-pre-gyp https download 
node-pre-gyp WARN Tried to download(404): https://fsevents-binaries.s3-us-west-2.amazonaws.com/v1.2.7/fse-v1.2.7-node-v72-darwin-x64.tar.gz 
node-pre-gyp WARN Pre-built binaries not found for [email protected] and [email protected] (node-v72 ABI, unknown) (falling back to source compile with node-gyp) 
node-pre-gyp WARN Pre-built binaries not installable for [email protected] and [email protected] (node-v72 ABI, unknown) (falling back to source compile with node-gyp) 
node-pre-gyp WARN Hit error Connection closed while downloading tarball file 
...