Add note about removed drush aliases in 1.x -> 2.x upgrade section

Problem/Motivation

This is a critical module for automated site deployment routines, where drush commands are often utilized. When upgrading from 1.x to 2.x, it is currently easy to miss that any usages of "drush csex" or "drush csim" need to be updated to their non-aliased versions ("config-split:export" and "drush config-split:import"), as this is not mentioned in the "Upgrade from version 1.x to 2.x" section on the module's project page.

Adding a note about this potentially site-breaking change would be very helpful to those of us upgrading from 1.x to 2.x.

Steps to reproduce

N/A.

Proposed resolution

Under the "Upgrade from version 1.x to 2.x" section on the module's project page, make it clear that the "csex" and "csim" drush command aliases have been removed in the 2.x branch and that any usages of them must be replaced by the non-aliased "config-split:export" and "config-split:import" commands.

Remaining tasks

Idea review, project page update.

User interface changes

None.

API changes

None.

Data model changes

None.