Adds engines 2.1.0 plugin.

git-svn-id: http://redmine.rubyforge.org/svn/trunk@1654 e93f8b46-1217-0410-a6f0-8f06a7374b81

vendor/plugins/engines/generators/plugin_migration/USAGE

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+Description:
+  The plugin migration generator assists in working with schema additions 
+  required by plugins. Instead of running migrations from plugins directly,
+  the generator creates a regular Rails migration which will be responsible
+  for migrating the plugins from their current version to the latest version
+  installed.
+  
+  This is important because the set of application migrations remains an
+  accurate record of the state of the database, even as plugins are installed
+  and removed during the development process.
+
+Example:
+  ./script/generate plugin_migration [<plugin_name> <another_plugin_name> ...]
+
+  This will generate:
+
+    RAILS_ROOT
+      |- db
+          |-migrate
+              |- xxx_plugin_migrations.rb
+
+  which contains the migrations for the given plugin(s).
+  
+  
+Advanced Usage:
+
+There may be situations where you need *complete* control over the migrations
+of plugins in your application, migrating a certainly plugin down to X, and
+another plugin up to Y, where neither X or Y are the latest migrations for those
+plugins.
+
+For those unfortunate few, I have two pieces of advice:
+
+ 1. Why? This is a code smell [http://c2.com/xp/CodeSmell.html].
+ 
+ 2. Well, OK. Don't panic. You can completely control plugin migrations by
+    creating your own migrations. To manually migrate a plugin to a specific
+    version, simply use
+    
+      Engines.plugins[:your_plugin_name].migrate(version)
+      
+    where version is the integer of the migration this plugin should end
+    up at.
+    
+With great power comes great responsibility. Use this wisely.