update welcome message

[Rich-Harris]

Not sure how the typo ('some feature might not work yet') escaped our notice this long, but I figured I'd take the opportunity to tighten up the welcome message a bit generally

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

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
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[Rich-Harris]

Fixed a couple of other rough onboarding edges

[Conduitry]

I know we've changed our mind at least a time or two about whether to include an adapter by default. What's behind this latest decision?

[Rich-Harris]

a) I just think it's a bit weird to make the Node adapter the default, b) running into that message when you first build makes it much clearer that you're supposed to select an adapter, and makes it more likely that someone will figure out how (and be aware of the various options). In the past I think we had to include one because it failed if you didn't

[Rich-Harris]

i am perturbed by these windows failures.