Added indexing section

README.md

@@ -138,6 +138,18 @@ class Person
 end
 ```
 
+### Indexing
+
+There are 3 ways to populate the Sphinx index: use the model class' `sphinx_indexer!` method, `Mongoid::Giza::Indexer.instance.index!` or `Mongoid::Giza::Indexer.instance.full_index`
+
+* **sphinx_indexer!:** Will execute the indexer program only on the indexes of the class.
+Does not regenerate dynamic indexes.
+* **index!:** Will execute the indexer program on all indexes.
+Does not regenerate dynamic indexes.
+* **full_index:** Will regenerate dynamic indexes and execute the indexer program on all indexes.
+
+This gem does not execute none of those automatically to let the you define what is the best reindexing strategy for your software.
+
 ### Searching
 
 Use the `search` block on the class that have the indexes where the search should run.