Salt minion allow to specify HTTP backend

This is useful when using proxies. The default Tornado backend does not
utilize proxy environment variables and isn't able to set no_proxy variable.

Change-Id: I4a51c6fc9abe65d46ed4f3adeb30f7a25337857e
diff --git a/README.rst b/README.rst
index 5f528fd..7ec1dfd 100644
--- a/README.rst
+++ b/README.rst
@@ -364,6 +364,17 @@
           host: 127.0.0.1
           port: 3128
 
+Salt minion to specify non-default HTTP backend. The default tornado backend
+does not respect HTTP proxy settings set as environment variables. This is
+useful for cases where you need to set no_proxy lists.
+
+.. code-block:: yaml
+
+    salt:
+      minion:
+        backend: urllib2
+
+
 Salt minion with PKI certificate authority (CA)
 
 .. literalinclude:: tests/pillar/minion_pki_ca.sls