add some missing docs for PKCS8SerializationBackend, update changelog
diff --git a/docs/hazmat/backends/interfaces.rst b/docs/hazmat/backends/interfaces.rst
index 78a35cd..6f24f90 100644
--- a/docs/hazmat/backends/interfaces.rst
+++ b/docs/hazmat/backends/interfaces.rst
@@ -454,3 +454,25 @@
:returns:
:class:`~cryptography.hazmat.primitives.interfaces.CMACContext`
+
+
+.. class:: PKCS8SerializationBackend
+
+ .. versionadded:: 0.5
+
+ A backend with methods for working with PKCS #8 key serialization.
+
+ .. method:: load_pkcs8_pem_private_key(data, password)
+
+ :param bytes data: PEM data to deserialize.
+
+ :param bytes password: The password to use if this data is encrypted.
+ Should be None if the data is not encrypted.
+
+ :return: A new instance of the appropriate private key or public key
+ that the serialized data contains.
+
+ :raises ValueError: If the data could not be deserialized correctly.
+
+ :raises cryptography.exceptions.UnsupportedAlgorithm: If the data is
+ encrypted with an unsupported algorithm.