Merge #1548: README: mention ellswift module

7454a53 README: mention ellswift module (Sebastian Falbesoner)

Pull request description:

ACKs for top commit:
  sipa:
    utACK 7454a53
  real-or-random:
    utACK 7454a53

Tree-SHA512: 50b00a08421740549c81df7320aad6c681bf8bd6c86ec3b22b48ea2127ecff63d6c0f9fe29a33708c8809bcd162717518ec82a17727f269a14430c82b8d70e3d

Author: real-or-random

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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