Add sign_x509_certificate to X509Backend interface

Add note about CertificateBuilder to the changelog
diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index 4506a46..f07a908 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -45,6 +45,13 @@
   * :class:`~cryptography.x509.KeyUsage`
   * :class:`~cryptography.x509.SubjectAlternativeName`
 
+* Add support for creating signed certificates with
+  :class:`~cryptography.x509.CertificateBuilder`. This includes support for
+  the following extensions
+
+  * :class:`~cryptography.x509.BasicConstraints`
+  * :class:`~cryptography.x509.SubjectAlternativeName`
+
 0.9.3 - 2015-07-09
 ~~~~~~~~~~~~~~~~~~
 
diff --git a/src/cryptography/hazmat/backends/interfaces.py b/src/cryptography/hazmat/backends/interfaces.py
index 4d378e6..49ccda1 100644
--- a/src/cryptography/hazmat/backends/interfaces.py
+++ b/src/cryptography/hazmat/backends/interfaces.py
@@ -280,6 +280,12 @@
         Create and sign an X.509 CSR from a CSR builder object.
         """
 
+    @abc.abstractmethod
+    def sign_x509_certificate(self, builder, private_key, algorithm):
+        """
+        Sign an X.509 Certificate from a CertificateBuilder object.
+        """
+
 
 @six.add_metaclass(abc.ABCMeta)
 class DHBackend(object):