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

Review URL: http://codereview.chromium.org/7198021

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8740 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

tools/oom_dump/README

@@ -16,7 +16,9 @@ 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: