Fix doc on dev requirements

README.md

@@ -17,9 +17,9 @@ We use django framework.  Install it from requirements with pip:
 
      $ pip install -r requirements/base.txt
 
-For development, install `dev.txt` requirements too:
+For development, install `dev.txt` instead:
 
-     $ pip install -r requirements/base.txt -r requirements/dev.txt
+     $ pip install -r requirements/dev.txt
 
 Set up configuration
 ====================