Checkin descriptive note

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@2765 91177308-0d34-0410-b5e6-96231b3b80d8

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+I checked in a new tool, primarily useful for debugging.  Given a module 
+and a function name, it extracts just the specified function from the 
+module, with a minimum of related cruft (global variables, function 
+prototypes, etc).
+
+This is useful because often something will die (for example SCCP 
+miscompiles one function of a large benchmark), and so you want to just 
+cut the testcase down to the one function that is being a problem.  In 
+this case, 'extract' eliminates all of the extraneous global variables, 
+type information, and functions that aren't neccesary, giving you 
+something simpler.
+
+This is just an FYI, because I've found it useful and thought you guys 
+might as well.
+