[functions] Root Readme does not contain an overview

[dackers86]

Issue Readme overview does not match other SDK overviews.

Step to follow

1. Navigate to the root README for the Dart functions repository.

Acceptance Criteria

• The intro for the README has been expanded to include a broader over view similar the the NodeJS functions SDK.

Cloud Functions is a hosted, private, and scalable Dart environment where you can run Dart code. The Dart Firebase SDK for Cloud Functions integrates the Firebase platform by letting you write code that responds to events and invokes functionality exposed by other Firebase features.