Merge branch 'hotfix/docu_fix'

Author: swayf

README.md

@@ -14,9 +14,10 @@ Install via NPM
 
 ```javascript
 Model.plugin(tree, {
-  pathSeparator : '#'    // Default path separator
-  onDelete :  'REPARENT' // Can be set to 'DELETE' or 'REPARENT'. Default: 'REPARENT'
-  numWorkers: 5          // Number of stream workers
+  pathSeparator : '#'              // Default path separator
+  onDelete :      'REPARENT'       // Can be set to 'DELETE' or 'REPARENT'. Default: 'REPARENT'
+  numWorkers:     5                // Number of stream workers
+  idType:         Schema.ObjectId  // Type used for _id. Can be, for example, String generated by shortid module
 })
 ```