minimal instructions to vue 3 adopters who use webpack

[joaomelo]

Problem: Since vue-template-compiler has no corresponding version to Vue 3, adopters who use Webpack can be confused by this text: "The reason vue-template-compiler has to be installed separately is so that you can individually specify its version. Every time a new version of vue is released, a corresponding version of vue-template-compiler is released together."

Proposal: This change suggests a small text to alert these users that they have to change the package and link to a more detailed instruction.

[Drincann]

That's a big help