ref(docs): Update README with more info on prerequisites

[andreiborza]

Small refactor to make it clearer what secrets need to be set.

[szokeasaurusrex]

I think this is a generally good refactor, but imo this is a bit too much information for what should be a simple guide to cover the basic use cases

[szokeasaurusrex]

The organization and Sentry URL are encoded in the org auth token. We recommend that users do not manually set these when they are using an org auth token. Users should only need to set the project slug.

Suggested change

	You also need to set your Organization and Project slugs and if you're using a self-hosted Sentry instance, provide the URL used to connect to Sentry via SENTRY_URL.
	You also need to set your project slug.

[andreiborza]

Does that hold true for self-hosted too?

Anyway, this isn't a simple docs/readme change as the action has code that depends on these. We can think of removing in another issue. Let's discuss this in our meeting.

[szokeasaurusrex]

Yes, it applies to self hosted (minus, some old self-hosted versions that did not encode the URL due to a bug). Sentry CLI (at least for the URL, not sure about the org) will ignore the manually provided values and use the ones encoded in the token, and it also issues a warning in case of a difference.

The whole point of encoding the URL and org in the token was to eliminate the need to manually specify them, so we should definitely remove any code that depends on these being manually provided from the GitHub action

[andreiborza]

As discussed, let's tackle this in #212

[szokeasaurusrex]

#212 seems to imply we would need to make these changes in a major release, but I think we can make them in a minor release, too

[andreiborza]

We can, but priority wise we want to tackle a new major first.