add chacha20 test vectors from RFC 7539 (#3918)

diff --git a/docs/development/test-vectors.rst b/docs/development/test-vectors.rst
index dc89caf..59dfdf9 100644
--- a/docs/development/test-vectors.rst
+++ b/docs/development/test-vectors.rst
@@ -440,6 +440,7 @@
 * CAST5 (ECB) from :rfc:`2144`.
 * CAST5 (CBC, CFB, OFB) generated by this project.
   See: :doc:`/development/custom-vectors/cast5`
+* ChaCha20 from :rfc:`7539`.
 * ChaCha20Poly1305 from :rfc:`7539`, `OpenSSL's evpciph.txt`_, and the
   `BoringSSL ChaCha20Poly1305 tests`_.
 * IDEA (ECB) from the `NESSIE IDEA vectors`_ created by `NESSIE`_.