Add steps to migrate existing app to remix-fastify

[swalker326]

I've updated lots of apps to remix-fastify and always have to remember how so I added the steps to the readme.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 4180a82

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[mcansh]

thanks for the PR @swalker326! i think i used to have something in the readme for migration, but removed it at some point, so thanks for bringing it back! i'll review this in a bit and merge :)