README.md in drive_env-0.3.1 vs README.md in drive_env-0.4.0

@@ -34,12 +34,14 @@
 
     $ gem install drive_env
 
 ## Usage
 
-First, you need to login Google Cloud Platform and enable APIs.
+First, you need to login Google Cloud Platform and enable APIs. We support authentication with `User account` and `Service account`.
 
+### User authentication
+
 1. Open [API library page](https://console.developers.google.com/apis/library)
 2. Create/Select project
 3. Enable "Google Drive API" and "Google Sheets API"
 4. Open [Credentials page](https://console.developers.google.com/apis/credentials) in the same project
 5. Click "Create credentials" and choose "OAuth client ID"
@@ -54,10 +56,27 @@
 $ drive_env auth login
 ```
 
 Now you have access token and refresh token, you can access to Google APIs.
 
+### Service account authentication
+
+1. Open [API library page](https://console.developers.google.com/apis/library)
+2. Create/Select project
+3. Enable "Google Drive API" and "Google Sheets API"
+4. Open [Credentials page](https://console.developers.google.com/apis/credentials) in the same project
+5. Click "Create credentials" and choose "Service account key"
+6. Save credentials as JSON file
+7. Set `GOOGLE_APPLICATION_CREDENTIALS` environment variables with the value of the downloaded JSON file path.
+  ```
+  $ export GOOGLE_APPLICATION_CREDENTIALS="/path/to/service_account_credential.json"
+  ```
+
+The application search `GOOGLE_APPLICATION_CREDENTIALS` first and then try to use user credential.
+
+### Access Spreadsheet
+
 Show Spreadsheet in Google Drive:
 
 ```
 $ drive_env spreadsheet show 'https://docs.google.com/spreadsheets/d/*********/edit#gid=0'
 ```
@@ -97,11 +116,11 @@
 $ your-ruby-application-with-dotenv-gem
 ```
 
 ## Development
 
-After checking out the repo, run `bin/setup` to install dependencies. Then, run `rake rspec` to run the tests. You can also run `bin/console` for an interactive prompt that will allow you to experiment.
+After checking out the repo, run `bin/setup` to install dependencies. Then, run `rake spec` to run the tests. You can also run `bin/console` for an interactive prompt that will allow you to experiment.
 
 To install this gem onto your local machine, run `bundle exec rake install`. To release a new version, update the version number in `version.rb`, and then run `bundle exec rake release`, which will create a git tag for the version, push git commits and tags, and push the `.gem` file to [rubygems.org](https://rubygems.org).
 
 ## Contributing
 
@@ -109,6 +128,5 @@
 
 
 ## License
 
 The gem is available as open source under the terms of the [MIT License](http://opensource.org/licenses/MIT).
-