Spectral: Clarify --ruleset requirement

[ryotrellim]

Relevant conversation: https://discord.com/channels/841794018173648916/859895506271600670/939506075525320714

Relevant tickets:
bundesAPI/deutschland#48
#1893
stoplightio/spectral-action#618
#1796

Goal

The first line of https://meta.stoplight.io/docs/spectral/ZG9jOjI1MTg1-spectral-cli will actually throw an error.
i.e., Running spectral lint petstore.yaml immediately after install will likely fail with No ruleset was found.

Solution

Add the two ways to get a ruleset.

Create a Ruleset
Spectral, being a generic YAML/JSON linter, needs a ruleset in order to be able to lint files. There are two ways to do this:

1. Run this comment to reference our predefined results:
   printf '{\n "extends": ["spectral:oas", "spectral:asyncapi"]\n}\n' > .spectral.json
2. Create your own ruleset. (Link to Custom ruleset topic.)

[P0lip]

@ryotrellim it's in the readme together with a snippet you can use, literally one of the first sentences you have there.

If you intend to lint an OpenAPI or AsyncAPI document, we have a few predefined rulesets you can extend to get Spectral up and running. To reference them, you can run the following command:

[pamgoodrich]

@P0lip : I believe you are saying the doc is correct. I was able to get this command to work locally:

spectral lint openweather.json

Once I had a ruleset named .spectral.yaml with this line at the top:

extends: spectral:oas

I can help come up with some clearer steps if that would help. Let me know.

[P0lip]

I meant the following segment. There's a command you can run that will get you started. It'll create a .spectral.json file which is everything you need to start linting.