Update user guide with Sphinx docs info (#4554)

docs/userguide/index.rst

@@ -27,3 +27,4 @@
     testing
     extending
     configuration
+    sphinx

docs/userguide/sphinx.rst

@@ -0,0 +1,16 @@
+.. _sphinx:
+
+==============================
+ Documenting Tasks with Sphinx
+==============================
+
+This document describes how auto-generate documentation for Tasks using Sphinx.
+
+
+--------------------------------
+ celery.contrib.sphinx
+--------------------------------
+
+.. automodule:: celery.contrib.sphinx
+    :members:
+    :noindex: