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is the currently recommended version, I'm guess that it's the Modular API and that the version 7 upgrade document is out of date.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

README の Example Use、Resources、Developer Guide セクションを docs/version-7-upgrade.md と比較し、特に Modular API が未完成であるという記述を確認します。現在推奨されている API をプロジェクトのメンテナーに確認してから、README とアップグレードガイドの内容を一致させ、矛盾する案内がなくなっていることを確認します。

索引モデルが issue の本文から書いたものです。