Added reference to index creation on mongoDB

README.md

@@ -34,6 +34,13 @@ class River
 end
 ```
 
+Generate indexes on MongoDB:
+
+```
+rake db:mongoid:create_indexes
+```
+
+
 Before we manipulate the data mongoid_spacial handles is what we call points.
 
 Points can be:
@@ -143,6 +150,14 @@ rivers = rivers.per(25).page(1)
 rivers.reset! # resets the object to it is original state right after query.
 ```
 
+Troubleshooting
+-------------
+
+**Mongo::OperationFailure: can't find special index: 2d**
+
+Indexes need to be created. Execute command: <code>rake db:mongoid:create_indexes</code>
+
+
 Thanks
 -----------
 * Thanks to Kristian Mandrup for creating the base of the gem and a few of the tests