Sapper: Document --legacy

[Rich-Harris]

https://github.com/sveltejs/sapper/issues/280

[benallfree]

https://github.com/sveltejs/sapper/issues/280#issuecomment-419588184 I thought the same thing, maybe --include-legacy would be clearer.

[kylecordes]

Here is an alternative idea. Rather than document this option, consider removing the option. Instead a "production" build would automatically include differential bundling. In the future when there are almost no browsers left that need the old code, an option could be introduced to --skip-legacy.

Or maybe by then the ES5 output could be removed, and no option (thus no documentation other than "of course Sapper does differential bundling, you need not even ask") would ever be needed.

[sw-yx]

@Conduitry - may i ask why this was transferred over from the docs repo?

i'm going to open up a WIP PR for this one and we can discuss documenting there

[Conduitry]

This is the docs repo now as well. https://sapper.svelte.dev/ lives in the site directory of this repo. Since Sapper was updated for Svelte 3, the other repo is for what's at https://sapper-legacy.svelte.dev/.

[sw-yx]

ok i've moved over the PR!

[diverent2]

Here's another idea: How about using --support-legacy instead?
IMO this clarifies that it's meant for browser support without getting too verbose or causing confusion.

Just an idea though :)

[sw-yx]

please open a new issue, i gave up on my pr