Add migration plan

Author: kevintang2022

RFC-0012-built-in-native-function-namespace-manager.md

@@ -94,6 +94,13 @@ this.memoizedFunctionsSupplier = Suppliers.memoize(this::bootstrapNamespace);
 
 ```
 
+## Adoption Plan
+
+In order to control rolling out this feature, add a new configuration file, like "etc/builtin-native-functions.properties". If the file exists, enable the feature. Otherwise, do not attempt to load any native functions as built in functions. Within the file, we can also add config that controls which type of functions use the worker implementation override or not. For example, only use worker implementation when there is a SQL implementation.
+
+Once constant folding is enabled to C++ evaluation, this feature can be fully enabled to override all coordinator implementations. However, if this use case is not needed anymore, the feature can be removed, and we can set native.default to be default namespace to just use the existing native function namespace manager.
+
+
 ## Test Plan
 
 1. Unit test to invoke a SQL function that has a C++ implementation