Add docs about plugin generator

Fixes #5418

docs/static/contributing-to-logstash.asciidoc

@@ -14,7 +14,7 @@ Since plugins can now be developed and deployed independently of the Logstash
 core, there are documents which guide you through the process of coding and
 deploying your own plugins:
 
-
+* <<plugin-generator,Generating a New Plugin>>
 * http://www.elasticsearch.org/guide/en/logstash/current/_how_to_write_a_logstash_input_plugin.html[How to write a Logstash input plugin]
 * http://www.elasticsearch.org/guide/en/logstash/current/_how_to_write_a_logstash_codec_plugin.html[How to write a Logstash codec plugin]
 * http://www.elasticsearch.org/guide/en/logstash/current/_how_to_write_a_logstash_filter_plugin.html[How to write a Logstash filter plugin]

docs/static/plugin-generator.asciidoc

@@ -0,0 +1,19 @@
+[[plugin-generator]]
+== Generating Plugins
+
+You can now create your own Logstash plugin in seconds! The generate subcommand of `bin/logstash-plugin` creates the foundation 
+for a new Logstash plugin with templatized files. It creates the right directory structure, gemspec files and dependencies so you 
+can start adding custom code process data with Logstash.
+
+**Example Usage**
+
+[source,sh]
+--------------------------------------------
+bin/logstash-plugin generate --type input --name xkcd --path ~/ws/elastic/plugins
+-------------------------------------------
+
+* `--type`: Type of plugin - input, filter, output and codec
+* `--name`: Name for the new plugin
+* `--path`: Directory path where the new plugin structure will be created. If not specified, it will be '
+created in the current directory.
+