Add notes in source code and readme to clarify setup.

[ipokkel]

Is your feature request related to a problem? Please describe.
We include an example in the source code that is commented out of how to declare the $pmprosm_sponsored_account_levels variable. Users sometimes may find this confusing a could possibly edit the source code and uncomment it.

Describe the solution you'd like
We may consider adding notes in the commented out sample and in the plugin's readme file that the plugin's source code should not be edited directly and that this variable should be declared in a customization plugin or the theme's functions file.

Additional context
Admins Only: Contextual Reference.