docs for explicit backend selection and document name attribute of backend
diff --git a/docs/contributing.rst b/docs/contributing.rst
index 8e32c36..3710abe 100644
--- a/docs/contributing.rst
+++ b/docs/contributing.rst
@@ -250,6 +250,16 @@
You may not have all the required Python versions installed, in which case you
will see one or more ``InterpreterNotFound`` errors.
+
+Explicit Backend Selection
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+While testing you may want to run tests against a subset of the backends that
+cryptography supports. Explicit backend selection can be done via the
+``--backend`` flag. This flag should be passed to ``py.test`` with a comma
+delimited list of backend names. To use it with ``tox`` you must pass it as
+``-- --backend``.
+
Building Documentation
~~~~~~~~~~~~~~~~~~~~~~