docs: update usage section in readme

[Ananya2001-an]

Please provide details about:

• What you're trying to do
  Currently in the usage section of readme we have told users to run npm init ava to get started with ava but it's a bit misleading since someone may not have created a node project yet and might directly run this command thinking that it would create a node project with ava as a dependency in it which in fact doesn't happen. Also after creating the node project and running this command will not add the line "type": "module" in the package.json file by default so we need to tell users to add it explicitly.

I think we should update the readme docs with this extra information for clarity on steps that users should follow.

[Ananya2001-an]

I would like to work on this if it's approved.