[Documentation] Document the order of formats preferred

[marmistrz]

README.md mentions a lot about determining precedence of formats when it comes to quality. There's no information about what order of formats is the default one (webm, mp4, etc.) and how it's changed by --prefer-free-formats.

[dstftw]

Fell free to file a PR.

[marmistrz]

I'd file if I knew at all. I'll do that if you point me where it's defined.

[dstftw]

Order of formats is an implementation detail and may change any time. --prefer-free-formats gives preference to free formats over non-free when several formats are identical according to quality metadata. Similarly, free formats order is an implementation detail.

[marmistrz]

Which formats arw considered free and which not?

[dstftw]

Royalty-free and non royalty-free correspondingly.

[marmistrz]

Is the list of royalty-free formats somewhere in the code?

Look, all my knowledge in this matter is that mp3 is patented. Just as most users'