Update readme file with source code information

[urmil22]

Title: docs: Update README to Point to facebook/flow Repository

Description:

This pull request addresses issue #120 by updating the README file to include a reference to the facebook/flow repository. This change clarifies that the flow-bin repository provides binary releases for Flow, while the source code is maintained in the facebook/flow repository.

Changes Made:

Added a new section in the README file to guide users to the source code repository.

Motivation and Context:

The flow-bin repository only contains binary releases, whereas the actual source code is hosted in the facebook/flow repository. This update ensures that users are directed to the correct repository for source code, contributing, and additional documentation.

Related Issue:

Fixes #120

Types of Changes:

Documentation update

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