Make install code run if users copy and paste (#260)

New Julia users may not understand the meaning of ] to enter the
package manager from the Julia REPL. Instead, start with code that
works when they copy and paste.

Author: clarkfitzg

README.md

@@ -11,7 +11,7 @@
 ## Installation
 
 ```jl
-]add Debugger
+julia> import Pkg; Pkg.add("Debugger")
 ```
 
 # Usage