Add docs for auto config reloading

Fixes #4847

docs/static/command-line-flags.asciidoc

@@ -56,6 +56,12 @@ Logstash has the following flags. You can use the `--help` flag to display this
   Logstash can read multiple config files from a directory. If you combine this
   flag with `--debug`, Logstash will log the combined config file, annotating the
   individual config blocks with the source file it came from.
+  
+-r, --[no-]auto-reload
+  Monitor configuration changes and reload the configuration whenever it is changed.
+  
+--reload-interval RELOAD_INTERVAL
+  Specifies how often Logstash checks the config files for changes. The default is every 3 seconds.
 
 -h, --help
   Print help

docs/static/reloading-config.asciidoc

@@ -0,0 +1,43 @@
+[[reloading-config]]
+=== Reloading the Config File
+
+Starting with Logstash 2.3, you can set Logstash to detect and reload configuration
+changes automatically.
+
+To enable automatic config reloading, start Logstash with the `--auto-reload` (or `-r`)
+command-line option specified. For example:
+
+[source,shell]
+----------------------------------
+bin/logstash –f apache.config --auto-reload
+----------------------------------
+
+NOTE: The `--auto-reload` option is not available when you specify the `-e` flag to pass
+in  configuration settings from the command-line.
+
+By default, Logstash checks for configuration changes every 3 seconds. To change this interval,
+use the `--reload-interval <seconds>` option,  where `seconds` specifies how often Logstash
+checks the config files for changes. 
+
+If Logstash is already running without auto-reload enabled, you can force Logstash to
+reload the config file and restart the pipeline by sending a SIGHUP (signal hangup) to the
+process running Logstash. For example:
+
+[source,shell]
+----------------------------------
+kill -1 14175
+----------------------------------
+
+Where 14175 is the ID of the process running Logstash.
+
+==== How Automatic Config Reloading Works
+
+When Logstash detects a change in a config file, it stops the current pipeline by stopping
+all inputs, and it attempts to create a new pipeline that uses the updated configuration.
+After validating the syntax of the new configuration, Logstash verifies that all inputs
+and outputs can be initialized (for example, that all required ports are open). If the checks
+are successful, Logstash swaps the existing pipeline with the new pipeline. If the checks
+fail, the old pipeline continues to function, and the errors are propagated to the console.
+
+During automatic config reloading, the JVM is not restarted. The creating and swapping of
+pipelines all happens within the same process.