Updated README.md

[estrellajm]

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Checklist

• Issue number for this PR: #nnn (required)
• Docs included?: (yes/no; required for all API/functional changes)
• Test units included?: (yes/no; required)
• In a clean directory, yarn install, yarn test run successfully? (yes/no; required) No, ran into issues as it could not find several modules in '@angular/fire/schematics'. What node version should I should I be using? I had v16.15.1, but it didn't let me install. ng-package error.

Description

Updated README.md to include information on the environment file & provider section.

Code sample