Document how to get a random number as an integer, fixes #2190
diff --git a/docs/random-numbers.rst b/docs/random-numbers.rst
index 8b119a3..5f94bf1 100644
--- a/docs/random-numbers.rst
+++ b/docs/random-numbers.rst
@@ -21,4 +21,12 @@
This will use ``/dev/urandom`` on UNIX platforms, and ``CryptGenRandom`` on
Windows.
+If you need your random number as an integer (for example, for
+:meth:`~cryptography.x509.CertificateBuilder.serial_number`), you can use
+``int.from_bytes`` to convert the result of ``os.urandom``:
+
+.. code-block:: pycon
+
+ >>> serial = int.from_bytes(os.urandom(20), byteorder="big")
+
.. _`always use your operating system's provided random number generator`: http://sockpuppet.org/blog/2014/02/25/safely-generate-random-numbers/
