Add example in Readme for the Oauth2.0 flow

[duykhoa]

This pull request adds an example for the usage of this library in the Oauth 2.0 flow, it is related to the issue #81 .

[mtomov]

#81 (comment)

This solution definitely works. What's different are a few additional attributes:

secrets = Google::APIClient::ClientSecrets.new(
      {
        "web" =>
          {
            "access_token" => current_user.token,
            "refresh_token" => current_user.refresh_token,
            "client_id" => Rails.application.secrets[:google_client_id],
            "client_secret" => Rails.application.secrets[:google_secret]
          }
      }

maybe it's good to clarify if/when those are needed as well.

[kirrg001]

How to install the client secret package with their new generated package system?
"google-api-client" is deprecated.

[dazuma]

It's been moved to google-apis-core.

[bajajneha27]

Hi, thank you for raising this PR and apologies for long delay delay in responding. At the moment, we won't be able to address this but we're definitely going to work on it. I've created another issue to keep track of it, you can follow it #367
I'm closing this PR for now. Please feel free to re-open it in case you have any other concern.