add X25519 test vectors from RFC 7748 section 5.2 (#3685)

diff --git a/docs/development/test-vectors.rst b/docs/development/test-vectors.rst
index ffb918c..55175f5 100644
--- a/docs/development/test-vectors.rst
+++ b/docs/development/test-vectors.rst
@@ -35,6 +35,7 @@
   * `Botan's ECC private keys`_.
 * `asymmetric/public/PKCS1/dsa.pub.pem`_ is a PKCS1 DSA public key from the
   Ruby test suite.
+* X25519 test vectors from :rfc:`7748`.
 
 
 Custom asymmetric vectors