Add documentation about plugin limitations

src/site/apt/index.apt

@@ -9,6 +9,30 @@
 
 Properties Maven Plugin
 
+* Plugin limitations
+
+  Maven load project model (from pom.xml) for project and all submodules at startup time.
+  During project reading all available properties are resolved from projects and settings properties.
+
+  This plugin (as all other) is executed in the later phase - when project model is already build in memory.
+
+  So properties read from external files by this plugin can not by used in project definitions in items like
+  <<<\<goal\>>>>, <<<\<version\>>>> and so on.
+
+  Properties read by plugin in one module are not propagated to other modules or child projects.
+
+  Properties are only available for other plugins in runtime like for <<<maven-resource-plugin>>> for filtering resources.
+
+  In order to resolve such issue new features must by implement in Maven itself, like:
+
+  * {{{https://issues.apache.org/jira/browse/MNG-7833}MNG-7833}}
+
+  * {{{https://issues.apache.org/jira/browse/MNG-7914}MNG-7914}}
+
+  []
+
+* Plugin Overview
+
   The Properties Maven Plugin is here to make life a little easier when dealing
   with properties.  It provides goals to read properties from files and URLs and write
   properties to files, and also to set system properties.
@@ -24,7 +48,7 @@ Properties Maven Plugin
 ----------------------------------------
 
   and invoke the {{{./read-project-properties-mojo.html}properties:read-project-properties}}
-  goal, it would be the same as declaring the following in your <pom.xml>:
+  goal, it would be similar (with described limitations) as declaring the following in your <pom.xml>:
 
 ----------------------------------------
   <properties>
@@ -34,10 +58,7 @@ Properties Maven Plugin
 ----------------------------------------
 
   The properties loaded from files or URLs can be used to filter resources
-  differently for different environments.  Those using Spring's
-  {{{http://www.springframework.org/docs/api/org/springframework/beans/factory/config/PropertyResourceConfigurer.html}PropertyPlaceholderConfigurer}}
-  can see how these goals can be useful. Note that the URL format accepts
-  Spring's classpath: style pseudo-URL syntax.
+  differently for different environments.
 
   Optionally, the plugin can process default values within property placeholders.