Doc: add maintainance warning on README

[NicolasRitouet]

Description

Following this discussion, I've added a note at the top of the readme to warn visitors that the project is not actively maintained anymore.

@deployd/developers wdyt ?

Motivation and Context

There's hasn't been any new feature or new important development in a long time. Visitors should be aware of this.

Maybe we could also add a list of alternatives.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[rgolea]

So sad 😢

[andreialecu]

Alternatives:
https://nestjs.com/
https://feathersjs.com/
https://moleculer.services/
https://serverless.com/