README: Add how to actually create a project

[ppo]

Nothing in the README about how to actually create a project using svelte-starter-kit?!? 🤷‍♂️
Easy to do and always a good idea to mention.

Like for ads, my brain don't pay much attention to the common layout so I didn't see the "Public template" and "use this template" button.

I was expecting something like npx svelte-starter-kit my-app.
Not having to clone the repo… that's well what we must do if we don't want to create a new repo on GitHub, right?

[ppo]

Source: GitHub doc “Creating a repository from a template”

Creating a repository from a template is similar to forking a repository, but there are important differences:

• A new fork includes the entire commit history of the parent repository, while a repository created from a template starts with a single commit.
• Commits to a fork don't appear in your contributions graph, while commits to a repository created from a template do appear in your contribution graph.
• A fork can be a temporary way to contribute code to an existing project, while creating a repository from a template starts a new project quickly.