Improve documentation on using custom CSS

[elvarb]

Was debugging why my custom css was not working.

The plugin creates a css file called supercharged-links-gen.css into the snippets folder. The contents of this file is completely managed by the plugin.

What I did was create a file called supercharged-links-custom.css where my custom css was included.

This did not work because the css files are applied in order, this meant that my custom file was applied first and then the settings were overwritten by the gen file.

To fix this the custom file has to be ordered after the after, like naming it supercharged-links-xcustom.css