Add a new Mode interface to document mode.name and start on some prose docs for interfaces.
diff --git a/docs/hazmat/primitives/index.rst b/docs/hazmat/primitives/index.rst
index c81018a..614c414 100644
--- a/docs/hazmat/primitives/index.rst
+++ b/docs/hazmat/primitives/index.rst
@@ -10,3 +10,4 @@
     hmac
     symmetric-encryption
     padding
+    interfaces