update readme

README.md

@@ -157,6 +157,44 @@ and in plugin respository you must have a plugin.properties file with the below
 
 For more information please see the demo sources.
 
+Plugin lifecycle
+--------------------------
+Each plugin passes through a pre-defined set of states. [PluginState](https://github.com/decebals/pf4j/blob/master/pf4j/src/main/java/ro/fortsoft/pf4j/PluginState.java) defines all possible states.   
+The primary plugin states are:
+- CREATED
+- DISABLED
+- STARTED
+- STOPPED
+
+The DefaultPluginManager contains the following logic:
+
+1. all plugins are resolved & loaded
+2. *DISABLED* plugins are NOT automatically *STARTED* by pf4j in `startPlugins()` BUT you may manually start (and therefore enable) a *DISABLED* plugin by calling `startPlugin(pluginId)` instead of `enablePlugin(pluginId)` + `startPlugin(pluginId)`
+3. only *STARTED* plugins may contribute extensions. Any other state should not be considered ready to contribute an extension to the running system.
+
+The differences between a DISABLED plugin and a STARTED plugin are:
+1. a STARTED plugin has executed Plugin.start(), a DISABLED plugin has not
+2. a STARTED plugin may contribute extension instances, a DISABLED plugin may not
+
+DISABLED plugins still have valid class loaders and their classes can be manually
+loaded and explored, but the resource loading - which is important for inspection - 
+has been handicapped by the DISABLED check.
+
+As integrators of pf4j evolve their extension APIs it will become
+a requirement to specify a minimum system version for loading plugins.  
+Loading & starting a newer plugin on an older system could result in
+runtime failures due to method signature changes or other class
+differences.  
+For this reason was added a manifest attribute (in PluginDescriptor) to specify a 'requires' version
+which is a minimum system version. Also DefaultPluginManager contains a method to
+specify the system version of the plugin manager and the logic to disable
+plugins on load if the system version is too old (if you want total control, please override isPluginValid() method). This works for both
+loadPlugins() and loadPlugin().  
+
+__PluginStateListener__ defines the interface for an object that listens to plugin state changes. You can use ```addPluginStateListener(PluginStateListener listener)``` and ```removePluginStateListener(PluginStateListener listener)``` from PluginManager if you want to add or remove a plugin state listener.  
+
+Your application, as a PF4J consumer, has full control over each plugin (state). So, you can load, unload, enable, disable, start, stop and delete a certain plugin using PluginManager (programmatically).
+
 Development mode
 --------------------------
 PF4J can run in two modes: **DEVELOPMENT** and **DEPLOYMENT**.