Add peer dependency to User Event docs

[nickserv]

User Event peer depends on DOM Testing Library, so it needs to be manually installed, as stated in the readme.

[MatanBobi]

This looks good for now, but I'm trying to understand why only user-event defines a peerDep unlike all the other packages that have testing-library/dom as a dependency.

[nickserv]

There's rationale here: testing-library/user-event#135

@MatanBobi @kentcdodds @ph-fritsche Do you have any thoughts on this? I think it's a bit surprising that User Event still uses peer dependencies, while framework libraries like React Testing Library use regular dependencies. Does User Event rely on deduping DOM Testing Library in ways that React Testing Library does not? If the only issue is allowing more flexible dependency resolution, we could just put the >= version range on @testing-library/dom in dependencies.

Anyway I will merge this for now, since it's currently still a peer dependency, and the docs should be up to date with the readme.

[ph-fritsche]

I think that another copy of @testing-library/dom might cause problems with the config.
When importing e.g. @testing-library/react it automatically replaces the actWrapper and eventWrapper.

If @testing-library/dom imports inside @testing-library/userEvent resolves to another copy, it would be one with default config.

For the same reason projects should not add @testing-library/dom to their dependencies when working with one of the framework packages.

[kentcdodds]

The reason is pretty simple actually.

People using @testing-library/react don't need to think about using or installing @testing-library/dom because it's a wrapper, but the people using @testing-library/user-event need to have @testing-library/dom installed (either directly or transitively via @testing-library/react) because it's a companion utility, not a wrapper.

[nickserv]

Reverting this in #1329 (explanation).