Add design proposal for restore hooks

Signed-off-by: Marc Campbell <marc.e.campbell@gmail.com>

design/restore-hooks.md

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+# Restore Hooks
+
+Velero supports Backup Hooks to execute commands at before and/or after a backup.
+This enables a user to, among other things, prepare data to be backed up.
+An example of this would be to attach an empty volume to a Postgres pod, use a backup hook to execute `pg_dump` from the data volume, and back up the volume containing the export.
+The problem is that there's no easy or automated way to include an automated restore process.
+After a restore with the example configuration above, the postgres pod will be empty, but there will be a need to manually exec in and run `pg_restore`.
+This is not specific to postgres backups, but any many database engines and other applications that have application-specific utilities to back up and restore the data.
+This document proposes a solution that allows a user to specify Restore Hooks, much like Backup Hooks, that can be executed during the restore process.
+
+## Goals
+
+- Enable custom commands to be run during a restore in order to mirror the commands that are available to the backup process.
+
+## Non Goals
+
+- Handling any application specific scenarios (postgres, mongo, etc)
+
+## Background
+
+(See introduction)
+
+## High-Level Design
+
+The Restore spec
+
+## Detailed Design
+
+
+## Alternatives Considered
+
+
+## Security Considerations
+
+N/A