Docs: Serving WSGI app modules from Gunicorn (#1817)

docs/source/custom.rst

@@ -14,3 +14,37 @@ a custom Application:
 
 .. literalinclude:: ../../examples/standalone_app.py
     :lines: 11-60
+
+Direct Usage of Existing WSGI Apps
+----------------------------------
+
+If necessary, you can run Gunicorn straight from Python, allowing you to
+specify a WSGI-compatible application at runtime. This can be handy for
+rolling deploys or in the case of using PEX files to deploy your application,
+as the app and Gunicorn can be bundled in the same PEX file. Gunicorn has
+this functionality built-in as a first class citizen known as
+:class:`gunicorn.app.wsgiapp`. This can be used to run WSGI-compatible app
+instances such as those produced by Flask or Django. Assuming your WSGI API
+package is *exampleapi*, and your application instance is *app*, this is all
+you need to get going::
+
+    gunicorn.app.wsgiapp exampleapi:app
+
+This command will work with any Gunicorn CLI parameters or a config file - just
+pass them along as if you're directly giving them to Gunicorn:
+
+.. code-block:: bash
+   # Custom parameters
+   $ python gunicorn.app.wsgiapp exampleapi:app --bind=0.0.0.0:8081 --workers=4
+   # Using a config file
+   $ python gunicorn.app.wsgiapp exampleapi:app -c config.py
+    
+Note for those using PEX: use ``-c gunicorn`` as your entry at build
+time, and your compiled app should work with the entry point passed to it at
+run time. 
+
+.. code-block:: bash
+   # Generic pex build command via bash from root of exampleapi project
+   $ pex . -v -c gunicorn -o compiledapp.pex
+   # Running it
+   ./compiledapp.pex exampleapi:app -c gunicorn_config.py