Readme: Add beginner-friendly info about hlgroups

[codevogel]

Makes the part about modifying hlgroups a little more beginner friendly.

Changes this:

You can also conveniently add/modify hlgroups using the config.overrides option.

To this:

You can also conveniently add/modify any hlgroups using the config.overrides option , allowing you to customize the looks of specific built-in elements, or any other external plugins that provides hlgroups. (See :help highlight for more information on hlgroups.)