npm readmes should mention steps to take when `types` is set explicitly in the TS config

[benediktwerner]

When types is set explicitly in the TS config, the two simple installation steps described in the npm readmes (e.g. for @types/web) aren't enough. You also need to add "web" to the types list. This should be mentioned in the READMEs.

[orta]

Yeah, makes sense - with TS 4.5 we'll have a much cleaner setup process too but adding that to the README can probably happen in a different PR.