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
 ~~~~~~~~~~~~~~~~~~