Put some docs for default_backend in a place.
diff --git a/docs/hazmat/bindings/index.rst b/docs/hazmat/bindings/index.rst
index 11355bf..746f459 100644
--- a/docs/hazmat/bindings/index.rst
+++ b/docs/hazmat/bindings/index.rst
@@ -8,3 +8,27 @@
openssl
interfaces
+
+
+Getting a Backend Provider
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. currentmodule:: cryptography.hazmat.bindings
+
+``cryptography`` aims to support multiple backends to ensure it can provide
+the widest number of supported cryptographic algorithms as well as supporting
+platform specific implementations.
+
+You can get the default backend by calling
+:func:`~default_backend`.
+
+The default backend will change over time as we implement new backends and
+the libraries we use in those backends changes.
+
+
+.. function:: default_backend()
+
+ :returns: An object that provides at least
+ :class:`~interfaces.CipherBackend`, :class:`~interfaces.HashBackend`, and
+ :class:`~interfaces.HMACBackend`.
+