fixed read me file for updated Pkg keyword use (JuliaLang#714)

`Pkg.*` keywords don't work without importing 'Pkg'. Updated read-me so that there's no confusion.

Author: nilesh-patil

README.md

@@ -23,6 +23,7 @@ or later* and run the installer.  Then run the Julia application
 (double-click on it); a window with a `julia>` prompt will appear.  At
 the prompt, type:
 ```julia
+using Pkg
 Pkg.add("IJulia")
 ```
 to install IJulia.
@@ -310,4 +311,4 @@ Restart the notebook and look for the error message when IJulia dies.
 the terminal where you launched `jupyter`).
 
 When you are done, set `ENV["IJULIA_DEBUG"]=false` and re-run
-`Pkg.build("IJulia")` to turn off the debugging output.
+`Pkg.build("IJulia")` to turn off the debugging output.