Add note about VitePress to README.md

[KnorpelSenf]

Before submitting the PR, please make sure you do the following

• [x] Read the Contributing Guidelines.
• [x] Provide a description in this PR that addresses what the PR is solving. If this PR is going to solve an existing issue, please reference the issue (e.g. close #123).

What is the purpose of this pull request?

• [ ] Bug fix
• [ ] New feature
• [x] Documentation update
• [ ] Other

Description

VitePress is so much better already, in so many respects. We recently migrated our highly customised site with ~370 pages and have not regretted it since. Given the slow development speed of VuePress, it feels like the responsible thing to do to make people aware of VitePress, so that they don't end up being stuck here.

I would understand it if you don't want to use VuePress as a vehicle to advertise the competing VitePress project, but IMHO the benefits of VitePress outweigh any concerns regarding fairness—especially because VitePress is more of an upgrade and less or a competitor to VuePress.

[meteorlxy]

Thanks, but there is already a tip in the v1 repo.