docs(readme): soften language around disabling purgecss

[smackfu]

Description

Soften up the language around disabling purgecss.

Motivation

Unless I'm misunderstanding, "the effect of disabling will have a negative impact on performance" does not seem accurate to me. If a module uses minimal CSS, and some of it is being incorrectly stripped out by purgecss, disabling purgecss seems like a straightforward way to fix the issue, and scary language doesn't help here.

Types of changes

Check boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation (adding or updating documentation)
• Dependency update
• Security update

Checklist

Check boxes that apply:

• My code follows the code style of this project.
• My change requires a change to the documentation and I have updated the documentation accordingly.
• These changes should be applied to a maintenance branch.
• I have added the Apache 2.0 license header to any new files created.