New config structure docs

[daniilsapa]

Resolves #89

Hi @illright

I added several new sections to README to describe the new structure, its concepts, and how to work with it. Also, I added a migration guide in a separate file, as you requested.

Let me know if you have any suggestions/corrections. Thank you in advance!

[illright]

Thanks for the detailed write-up!

One major concern I have here is the size of the README. Ideally, I'd like to keep the README short, and have a separate resource for plugin authors and people who want to quickly write a rule or two. In the near future, it will probably amount to a documentation site. That's why I don't think we should go into details about concepts like GlobArray, Config Object, etc. These are implementation details and terms rather than public facing components, in my opinion.

So maybe we can just keep the README to one example that includes fsd.configs.recommended, a global ignore, and then a change of severity for a certain glob?

As for the docs about writing plugins, I think we should hold on with this for now. Since we still have some technical and design issues to iron out (mostly on my side, with the toolkit stuff), I don't think we're ready to write docs just yet.

[daniilsapa]

I fixed all the issues. I decided not to delete the examples completely and moved them to a separate page.

[illright]

Sorry for not finding these comments before, the fixes for the previous comments are good, thanks!

[illright]

Actually, when reading the README in rendered Markdown, I noticed something else. One of them is a thing that is straight-up no longer true, others are simply clarifications

[daniilsapa]

Yes, your suggestions were completely reasonable, so I committed them

[daniilsapa]

Also, I thought about merging this PR last, because:

• While we are buttoning up the new config there can be slight changes to the documentation (e.g. what we decided about GlobalIgnores today)
• Let's not confuse people with documentation for features that haven't been released yet.

[illright]

Makes sense. Let's wait with this then

[daniilsapa]

@illright
FYI I'm going to update this PR with some information on how our glob matching logic works. Let me know if we need to add anything else.