Adding install.sh script

Copying large commands out of a README is slightly more daunting for new
users (or was for me, anyway).

README.md

@@ -22,10 +22,7 @@ version is 4.3.17.
   3. Create a new Zsh configuration by copying the Zsh configuration files
      provided:
 
-        setopt EXTENDED_GLOB
-        for rcfile in "${ZDOTDIR:-$HOME}"/.zprezto/runcoms/^README.md(.N); do
-          ln -s "$rcfile" "${ZDOTDIR:-$HOME}/.${rcfile:t}"
-        done
+        ~/.zprezto/install.sh
 
   4. Set Zsh as your default shell:
 

install.sh

@@ -0,0 +1,8 @@
+#!/bin/zsh
+setopt EXTENDED_GLOB
+
+DIR=$(dirname $0)
+
+for rcfile in $DIR/runcoms/^README.md(.N); do
+  ln -s $rcfile "${ZDOTDIR:-$HOME}/.${rcfile:t}"
+done