Update the migration guide

Signed-off-by: yihuaf <yihuaf@unkies.org>

MirgationGuide.md

@@ -1,14 +1,20 @@
+# Migration Guide
+
 This contains information for migrating library versions.
 
 ## V0.1.0 -> v0.2.0
 
 ### libcontainer
 
 - The `Rootless` struct has been re-named as `UserNamespaceConfig` , `RootlessIDMapper` has been re-named to `UserNamespaceIDMapper` , and correspondingly the `RootlessError` has been re-named to `UserNamespaceError` . This is due to the fact that the structure was to be used for containers when a new user namespace is to be created, and that is not strictly only for rootless uses. Accordingly, the fields of various structs has been updated to reflect this change :
-    - rootless (module name) -> user_ns
-    - Rootless::rootless_id_mapper -> UserNamespaceConfig::id_mapper
-    - LibcontainerError::Rootless -> LibcontainerError::UserNamespace
-    - ContainerBuilderImpl::rootless -> ContainerBuilderImpl::user_ns_config
-    - ContainerArgs::rootless -> ContainerArgs::user_ns_config
+  - rootless (module name) -> user_ns
+  - Rootless::rootless_id_mapper -> UserNamespaceConfig::id_mapper
+  - LibcontainerError::Rootless -> LibcontainerError::UserNamespace
+  - ContainerBuilderImpl::rootless -> ContainerBuilderImpl::user_ns_config
+  - ContainerArgs::rootless -> ContainerArgs::user_ns_config
+
+- Changes that will occur for properly running in rootless mode : TODO (@YJDoc2)
+
+- Executor now contains 2 methods for implementation. We introduce a `validate` step in addition to execute. The `validate` should validate the input OCI spec. The step runs after all the namespaces are entered and rootfs is pivoted.
 
-- Changes that will occur for properly running in rootless mode : TODO (@YJDoc2)
+- Executor is now composible instead of an array of executor. To implement multiple executor, create a new executor that runs all the executor. The users are now in control of how multiple executor are run.