Add disclaimer to the README about performance in large projects

[NullVoxPopuli]

Mentioned here: #6 (comment)
we now have both babel and template parsing happening twice.
For v2 addons this likely isn't a big deal because scope is usually quite constrained.
For apps though, components can reach the thousands, and production builds (where everything needs to be built) would take the biggest hit (today we always pay this cost because we build and bundle everything, but one day when we're using vite, we'll only be using what is needed to view the current page)

Longer term, we need to figure out a way to have all transpilation occur in a single pass. @ef4 has been investigating this with a fork of the babel parser.
(And if we are able to do everything in babel, we wouldn't need this rollup-plugin-glimmer-template-tag, so 🎉 fewer things!)