Mention secrets import in v2 doc?

[sb8244]

https://github.com/superfly/docs/edit/main/apps/migrate-to-v2.html.md

It may be worth mentioning that fly secrets import exists to bulk import a set of ENV. This is ideal for the migration actually, because there may be a lot of secrets.

[jsierles]

Thanks, that's a good idea. I've used a script in the past to clone secrets across apps: https://gist.github.com/jsierles/fa7208e07decc59a37ac789d2fb505aa

[catflydotio]

Thanks @sb8244, and @jsierles. Added the script to the doc.

Fixed by f8542fe