Update oom_dump README to indicate that people should build the correct V8 version.

tools/oom_dump/README

 oom_dump extracts useful information from Google Chrome OOM minidumps.
 
 To build one needs a google-breakpad checkout
 (http://code.google.com/p/google-breakpad/).
 
 First, one needs to build and install breakpad itself. For instructions
 check google-breakpad, but currently it's as easy as:
 
   ./configure
   make
   sudo make install
 
 (the catch: breakpad installs .so into /usr/local/lib, so you might
 need some additional tweaking to make it discoverable, for example,
 put a soft link into /usr/lib directory).
 
 Next step is to build v8.  Note: you should build x64 version of v8,
 if you're on 64-bit platform, otherwise you would get a link error when
-building oom_dump.
+building oom_dump. Also, if you are testing against an older version of chrome
+you should build the corresponding version of V8 to make sure that the type-id 
+enum have the correct values.
 
 The last step is to build oom_dump itself.  The following command should work:
 
   cd <v8 working copy>/tools/oom_dump
   scons BREAKPAD_DIR=<path to google-breakpad working copy>
 
 (Additionally you can control v8 working copy dir, but the default should work.)
 
 If everything goes fine, oom_dump <path to minidump> should print
 some useful information about the OOM crash.
 
 Note: currently only 32-bit Windows minidumps are supported.
+