docs: Add removal of configFile option to migration guide (#266)

Author: Luca Forstner

MIGRATION.md

@@ -10,6 +10,7 @@ This document serves as a migration guide, documenting all breaking changes betw
 - `@sentry/bundler-plugin-core` will no longer export the individual plugins but a factory function to create them.
 - Removed `customHeader` option in favor of `headers` option which allows for multiple headers to be attached to outgoing requests.
 - The `cliBinaryExists` function was renamed to `sentryCliBinaryExists`
+- Removed the `configFile` option. Options should now be set explicitly or via environment variables.
 
 ## Upgrading from 1.x to 2.x (Webpack Plugin Only)