Give instructions on how to install readline [Fixes #556]

lib/pry.rb

@@ -168,13 +168,20 @@ end
 
 require "method_source"
 require 'shellwords'
-require "readline"
 require "stringio"
 require "coderay"
 require "optparse"
 require "slop"
 require "rbconfig"
 
+begin
+  require 'readline'
+rescue LoadError
+  warn "You're running a version of ruby with no Readline support"
+  warn "Please `gem install rb-readline` or recompile ruby --with-readline."
+  exit!
+end
+
 if Pry::Helpers::BaseHelpers.jruby?
   begin
     require 'ffi'