Rewrite README to reflect fetching pin

It's a bad idea to fetch dependencies as part of your build process.
This makes it harder to package properly because:
* It is not obvious what you really depend on, so packagers can miss
  deps.
* Some build environments do not have network access
* It becomes more difficult to upgrade the dependency

For Pin in particular it makes sense to have a fetch mechanism since it
is not installable, but putting it in the Makefile hides this from the
user.

README.md

@@ -23,4 +23,10 @@ We recommend using OPAM for building SymFuzz.
 
 4. Build SymFuzz
 
-   make
+   1. Download and unpack pin
+
+     ./fetch-pin.sh
+
+   2. Build SymFuzz
+
+     make

fetch-pin.sh

@@ -0,0 +1,5 @@
+#!/bin/sh
+wget 'http://software.intel.com/sites/landingpage/pintool/downloads/pin-2.14-67254-gcc.4.4.7-linux.tar.gz'
+tar xfz pin-2.14-67254-gcc.4.4.7-linux.tar.gz
+mv pin-2.14-67254-gcc.4.4.7-linux pin
+rm pin-2.14-67254-gcc.4.4.7-linux.tar.gz