Update readme.

README.md

@@ -69,7 +69,7 @@ implementation:
 * [x] [One-time authentication](https://doc.libsodium.org/advanced/poly1305) (`crypto_onetimeauth_*`)
 * [x] [Sealed boxes](https://doc.libsodium.org/public-key_cryptography/sealed_boxes)
 * [x] [Key derivation](https://doc.libsodium.org/key_derivation) (`crypto_kdf_*`)
-* [ ] [Key exchange](https://doc.libsodium.org/key_exchange) (`crypto_kx_*`)
+* [x] [Key exchange](https://doc.libsodium.org/key_exchange) (`crypto_kx_*`)
 * [ ] [Short-input hashing](https://doc.libsodium.org/hashing/short-input_hashing)
 * [ ] [Password hashing](https://doc.libsodium.org/password_hashing/default_phf)
 * [ ] [Public-key signatures](https://doc.libsodium.org/public-key_cryptography/public-key_signatures)