Preserve custom processors/NARs

Describes the steps to preserve custom processors/NARs.

If you have created any custom NARs, a best practice is to store and reference them in a centralized location.

On each source node:

Create a second library directory called custom_lib. For example: /opt/configuration_resources/custom_lib.
Move your custom NARs to this new directory.
Add a new line to the nifi.properties file to specify this new lib directory. For example:
```
nifi.nar.library.directory.custom=/opt/configuration_resources/custom_lib
```

You will perform a similar procedure on the destination cluster nodes later in the migration process.