README.md in source_position-0.1.0 vs README.md in source_position-0.2.0

@@ -6,9 +6,11 @@
 [![Dependency Status](https://gemnasium.com/linjunpop/source_position.png)](https://gemnasium.com/linjunpop/source_position)
 [![Code Climate](https://codeclimate.com/badge.png)](https://codeclimate.com/github/linjunpop/source_position)
 
 Tested on 1.8.7, 1.9.3, ree, rbx-18mode, rbx-19mode
 
+If you are using 1.8.7 or ree, you have to install [ruby18_source_location].
+
 ## Installation
 
 Add this line to your application's Gemfile:
 
     gem 'source_position'