Update documentation regarding IE11 support

[dfdeagle47]

Thank you @tbleckert for this awesome lib. Unfortunately, the lack of IE11 support out of the box tripped me up. As mentioned https://github.com/tbleckert/react-select-search/issues/96, it might be a good idea to document how to use this lib with IE11 support to avoid any unexpected issues.

This adds a section in the README highlighting the need to use the CommonJS build for IE11 support, instead of the default (ESM) build which specifically excludes IE11 support.

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[tbleckert]

@dfdeagle47 Awesome, thank you! Much appreciated 🙏