Recommended API Version

First, the [Example Use](https://github.com/angular/angularfire#developer-guide) section of the README has examples using the Modular API. Then, in the [Resources](https://github.com/angular/angularfire#resources) section of the README, we're first directed to the [version 7 upgrade guide](https://github.com/angular/angularfire/blob/master/docs/version-7-upgrade.md) that specifically mentions in bolded text:

> **Most developers can stop here for now as the new API isn't feature complete.**

Finally, the README then proceeds to the [Developer Guide](https://github.com/angular/angularfire#developer-guide) section that again links to the upgrade doc, but then specifically calls out that it assumes we're using the Modular API. Even the new issue instructions recommend that we read said Developer Guide, indicating that perhaps the Modular API should be what we're using.

While I would like a direct answer as to which is the currently recommended version, I'm guess that it's the Modular API and that the version 7 upgrade document is out of date. If that's correct, and the Modular API is now feature complete, then it should be updated to prevent this type of confusion.

So sánh các phần Example Use, Resources và Developer Guide trong README với docs/version-7-upgrade.md, đặc biệt là các phát biểu về việc Modular API chưa hoàn chỉnh. Xác nhận API hiện được khuyến nghị với các maintainer của dự án, sau đó làm cho README và hướng dẫn nâng cấp nhất quán với nhau, đồng thời kiểm tra rằng các hướng dẫn mâu thuẫn đã được loại bỏ.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.