Update README.md With Multi-Byte String Details

Consider adding some details about multi-byte string support in Twine to README.md.

README.md

@@ -210,6 +210,11 @@ $string->base64()->wrap(76, "\r\n", Twine\Config\Wrap::HARD);
 
 Additional details available in the full documentation at <https://twine.phlak.net>.
 
+Multi-Byte Strings
+---------
+
+Twine aims for multi-byte string compatibility by relying on PHP's [Multibyte String extension](https://www.php.net/manual/en/book.mbstring.php) (mbstring) to perform string operations. For this reason, the mbstring extension is required. Multi-byte strings include Unicode encodings such as UTF-8 and UCS-2.
+
 Changelog
 ---------