Add information in the main README

[brunomenezes]

📄 Context

• Reporting a typo.
• Reporting a documentation bug.
• Documentation improvement.
• Documentation feedback.

Is there a specific documentation page you are reporting?

Main README documentation

Additional context or description

It is important to provide information about the repository for whoever lands on the repository page.

As a starter:

• How the repository is structured.
• Links to the other packages that may or may not have their own README's.
• How we release new versions of the subgraph packages.