Improved docs for rounded arrow

[wallynm]

The default docs in tippyjs-react directs new developers to understand that they need to use Headless Tippy to achieve this config, turns out that after trying several options i found a simple way documented in the original repo.

To prevent this miss lead, i've tried to add this info so any new dev could easily understand how to implement this.
Any feedback very welcome, ty by the awesome work in this lib (:

[wallynm]

BTW, i'm not a english native speaker, some sentences may need improvement

[KutnerUri]

+1
for mentioning import 'tippy.js/dist/svg-arrow.css';

[sarahlshowers]

This worked perfectly. I spent too long trying to get this working. Thanks for opening this PR @wallynm
Since it's still an issue, I would love to see this merged into the docs.