Add plugin to common command USAGE (#41457)

Because the PluginCommand is defined with `hide_command!`, running
`bin/rails -h` hides the plugin command.

As an alternative to removing the `hide_command!` this adds the plugin
command to the USAGE. Unless it's an engine, because adding an engine to
an engine doesn't make sense.

Both the railtie and engine terms are used so a user can search for both.

[Petrik de Heus + Rafael Mendonça França + Jonathan Hefner]

railties/CHANGELOG.md

@@ -4,6 +4,10 @@
     the task description instead of the general Rake help.
 
     *Petrik de Heus*
+    
+*   Add missing `plugin new` command to help.
+
+    *Petrik de Heus
 
 *   Fix `config_for` error when there's only a shared root array.
 

railties/lib/rails/commands/help/USAGE

@@ -9,6 +9,7 @@ The most common rails commands are:
 <% unless engine? -%>
  new          Create a new Rails application. "rails new my_app" creates a
               new application called MyApp in "./my_app"
+ plugin new   Create a new Rails railtie or engine
 <% end -%>
 
 All commands can be run with -h (or --help) for more information.