Update README installation instructions

- The installation instructions indicate that
  react-hot-loader should be installed as a
  dev dependency, however, since it is used
  in production when checking if module.hot,
  it should be pulled in as a dependency with
  the save flag.

README.md

@@ -18,7 +18,7 @@ Check out [the Migration to 3.0 guide](https://github.com/gaearon/react-hot-load
 
 ## Installation
 
-`npm install --save-dev react-hot-loader@next`
+`npm install --save react-hot-loader@next`
 
 ## Usage