Documemt how to use custom icons

Problem/Motivation

In https://www.drupal.org/project/extlink/issues/3281593 ✨ Enable to customize an icon HTML output Needs work the module was updated to support more using custom icons. However, the module's readme was not updated.

Steps to reproduce

Check the module's Readme about customisations.

Proposed resolution

Add information about how to use custom icons.

Remaining tasks

Add information about how to use custom icons.

User interface changes

None.

API changes

None.

Data model changes

None.