Add stub Release Candidate migration guide

Only includes Vite 3 so far as other changes are pending upstream PRs (Markdown component, Astro.url, etc.)

Co-Authored-By: Fred K. Schott <fkschott@gmail.com>

src/pages/en/migrate.md

@@ -9,6 +9,22 @@ This guide will help you migrate from older versions of Astro to the latest vers
 
 Read the guide below for major highlights and instructions on how to handle breaking changes.
 
+## Astro 1.0 Release Candidate
+
+The Astro v1.0 Release Candidate (RC) introduces some changes that you should be aware of when migrating from beta or earlier releases. See below for more details.
+
+:::note
+The Release Candidate is not yet installed by default for most users, but you can try it today by running this command:
+
+```sh
+npm install astro@next--rc
+```
+:::
+
+### Updated: Vite 3
+
+Astro v1.0 RC has upgraded from Vite 2 to [Vite 3](https://vitejs.dev/). We've handled most of the upgrade for you inside of Astro, however some subtle Vite behaviors may still change between versions. Refer to the official [Vite Migration Guide](https://vitejs.dev/guide/migration.html#general-changes) if you run into trouble.
+
 ## Astro 1.0 Beta
 
 On April 4, 2022 we released the Astro 1.0 Beta! 🎉