add README to migration

schema/crdb/turn-boot-on-fault-into-auto-restart/README.adoc

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+= Overview
+
+This migration replaces the `omicron.public.instance.boot_on_fault` column,
+which is a `bool`, with a new `auto_restart_policy` column, which is an enum
+(`omicron.public.instance_auto_restart`). The new enum type will allow
+auto-restart policies other than "always" and "never" to be added in the future.
+Existing instance records are backfilled with the `all_failures` variant of
+`instance_auto_restart` if `boot_on_fault` is `true`, or `never` if
+`boot_on_fault` is `false`.
+
+The migration performs the following operations:
+
+1. `up01.sql` creates the `instance_auto_restart` enum.
+2. `up02.sql` adds a (nullable) `auto_restart_policy` column to the `instance`
+   table.
+3. `up03.sql` updates instance records by setting `auto_restart_policy` to
+   `all_failures` if `boot_on_fault` is `true`, or `never` if `boot_on_fault` is
+   `false`.
+4. Now that all instance records have a value for `auto_restart_policy`,
+   `up04.sql` makes the `auto_restart_policy` column non-null.
+5. Finally, `up05.sql` drops the now-defunct `boot_on_fault` column.