add signals doc

add a brief description of the signals handled by Gunicorn.

fix #319

docs/source/index.rst

@@ -41,6 +41,7 @@ Contents
     configure
     deploy
     design
+    signals
     community
     faq
     news

docs/source/signals.txt

@@ -0,0 +1,40 @@
+.. _signals:
+
+================
+Signals Handling
+================
+
+A brief description of the signals handled by Gunicorn, We also document the
+signales used internally by Gunicorn to communicate with the workers. With the
+exception of TTIN/TTOU the signals handling match the behaviour of `nginx
+<http://wiki.nginx.org/CommandLine>`.
+
+Master process
+==============
+
+- **TERM**, **INT**: Quick shutdown
+- **QUIT**: Graceful shutdwn. I waits for workers to finish their
+  current request before finishing until the *graceful timeout*.
+- **HUP**: Reload the configuration, start the new worker processes with a new
+  configuration and gracefully shutdown older workers. If the application is
+  not preloaded (using the ``--preload`` option), Gunicorn will also load the
+  new version.
+- **TTIN**: Increment the number of processes by one
+- **TTOU**: Decrement the nunber of processes by one
+- **USR1**: Reopen the log files
+- **USR2**: Upgrade the Gunicorn on the fly. A separate **QUIT** signal should
+  be used to kill the old process. This signal can also be used to use the new
+  versions of pre-loaded applications.
+- **WINCH**: Gracefully shutdown the worker processes when gunicorn is
+  daemonized.
+
+Worker process
+==============
+
+Sending signals directly to the worker processes should not normally be
+needed.  If the master process is running, any exited worker will be
+automatically respawned.
+
+- **TERM**, **INT**: Quick shutdown
+- **QUIT**: Graceful shutdown
+- **USR1**: Reopen the log files