README: mention ellswift module

Co-authored-by: Tim Ruffing <crypto@timruffing.de>

Author: theStack

README.md

@@ -20,6 +20,7 @@ Features:
 * Optional module for public key recovery.
 * Optional module for ECDH key exchange.
 * Optional module for Schnorr signatures according to [BIP-340](https://github.com/bitcoin/bips/blob/master/bip-0340.mediawiki).
+* Optional module for ElligatorSwift key exchange according to [BIP-324](https://github.com/bitcoin/bips/blob/master/bip-0324.mediawiki).
 
 Implementation details
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