docs: adding simple icons slugs as resource for contributors

[allankildare]

Description

Hi guys! In this PR I am improving the markdown contributing text, now anyone who wants to contribute has access to the list of slugs for the icons.

What do you think about this tip?

:bulb: Tip: You can check available icons for your <badge> in Simple Icons slugs.

[kohasummons]

Super one @allankildare! Could we also add the simple icons website? your thoughts?

[allankildare]

Yeah, for sure! I will make this adjustment.

[allankildare]

@kohasummons Adjustment done!