Update documentation to have CLI instructions before the usage

[iepsen]

Why?

This is a suggestion to improve the communication about the React Native CLI. As it works only with the new CLI, it seems to be a requirement instead of a note.

The goal here is to avoid issues like this one facebook/react-native#31054.

[emin93]

Wow, it has been already 4 months since you opened this PR. Thank you for the patience.

Generally I can see where your motivation comes from. Fun fact, when the new CLI was released this note has already been up there. And guess what, people still reported the same error over and over again... 😄

In the meantime the new CLI is also not so new anymore (out for over 2 years) and I feel it would be too distracting for the majority of developers that already use the latest CLI.

It's not like the info isn't there, it's just not at the beginning of the usage section. I think it's fair to expect anyone to read the whole README.md as it's quite small, especially if issues occur.

[emin93]

Having that said, I think I would be fine renaming the title from "Note on the Legacy CLI" to something like "Requirements". Feel free to reopen this PR if you agree with that.

[leotm]

Notatebly #80

Good stuff 👊

[emin93]

Actually, I like the simple changes to make it feel less like an optional "note". Which is why I reopened this PR.

We should also add the hint about the --ignore-existing flag for people using npx with the old CLI. (#80 (comment))

The only thing I wouldn't want to change is moving it above the actual usage for the reasons stated in the previous comment.

[iepsen]

Thanks for looking into this @emin93. I have now pushed two commits to move back the legacy CLI warning and added the --ignore-existing instructions.