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is the currently recommended version, I'm guess that it's the Modular API and that the version 7 upgrade document is out of date.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

Compara las secciones Example Use, Resources y Developer Guide del README con docs/version-7-upgrade.md, especialmente las afirmaciones sobre que la Modular API está incompleta. Confirma la API recomendada actualmente con los mantenedores del proyecto y, después, haz que el README y la guía de actualización sean coherentes, y verifica que se hayan eliminado las indicaciones contradictorias.

Escrito por el modelo de indexación a partir del texto del issue.