Documentation on Custom Arrows omits important implementation detail

[mateusriff]

Describe the bug
I could not add custom arrows to my carousel following the docs on npm. The arrows would simply not show up. This is because an important implementation detail is omitted: custom arrows need their own styling, else they will not show up, as per issue #293.

To Reproduce
Steps to reproduce the behavior:

1. Go to the Custom Arrows section of the package's npm docs.
2. Follow the implementation steps laid out on the section, not adding styles to your custom arrows.

Expected behavior
Custom arrows should show up (none do).

[mateusriff]

I propose this section of the docs should be updated. It should be made clear custom arrows need their own styles. I'd be glad to contribute. What do you guys think?