Addd high level introductory docs for all the libs

include/osgDB/Export

@@ -34,4 +34,12 @@
 	#  define OSGDB_EXPORT
 #endif
 
+/**
+
+\namespace osgDB
+
+The osgDB library provides support for reading and writing scene graphs, providing a plugin framework and file utility classes.  
+The plugin framework in centred around the osgDB::Registry, and allows plugins which provide specific file format support to be dynamically loaded on demand.
+*/
+
 #endif