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is the currently recommended version, I'm guess that it's the Modular API and that the version 7 upgrade document is out of date.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.

將 README 的 Example Use、Resources 和 Developer Guide 部分與 docs/version-7-upgrade.md 進行比較，尤其關注關於 Modular API 尚不完整的陳述。與專案維護者確認目前建議使用的 API，然後讓 README 和升級指南保持一致，並確認衝突的指引已經移除。

由索引模型根據 Issue 內容生成。