add cast5 (cbc, cfb, ofb) vector source info to docs
diff --git a/docs/development/test-vectors.rst b/docs/development/test-vectors.rst
index 2a071d7..3b8632e 100644
--- a/docs/development/test-vectors.rst
+++ b/docs/development/test-vectors.rst
@@ -54,8 +54,34 @@
* Camellia (ECB) from NTT's `Camellia page`_ as linked by `CRYPTREC`_.
* Camellia (CBC, CFB, OFB) from `OpenSSL's test vectors`_.
* CAST5 (ECB) from :rfc:`2144`.
+* CAST5 (CBC, CFB, OFB) generated by this project.
+ See: :doc:`/development/custom-vectors/cast5`
+Creating Test Vectors
+---------------------
+
+When official vectors are unavailable ``cryptography`` may choose to build
+its own using existing vectors as source material. Current custom vectors:
+
+.. toctree::
+ :maxdepth: 1
+
+ custom-vectors/cast5
+
+If official test vectors appear in the future the custom generated vectors
+should be discarded.
+
+Any vectors generated by this method must also be prefixed with the following
+header format (substituting the correct information):
+
+.. code-block:: python
+
+ # CAST5 CBC vectors built for https://github.com/pyca/cryptography
+ # Derived from the AESVS MMT test data for CBC
+ # Verified against the CommonCrypto and Go crypto packages
+ # Key Length : 128
+
.. _`NIST`: http://www.nist.gov/
.. _`IETF`: https://www.ietf.org/
.. _`NIST CAVP`: http://csrc.nist.gov/groups/STM/cavp/