Add reference to Flex Plugin Library in readme

[dremin]

Summary

As discussed :)

Checklist

• Updated documentation
• Requested one or more reviewers

[trogers-twilio]

@dremin Thank you Sam! Should we also direct the reader that they should remove any features from the Flex Project Template that overlap with the installed Plugin Library features to avoid Flex configuration conflicts, linking to our guide on how to remove features from the Template?

[trogers-twilio]

This looks great! Thank you for these documentation updates and encouraging Flex users to leverage the native Plugin Library features when possible.