Add note about what this is

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@4581 91177308-0d34-0410-b5e6-96231b3b80d8

runtime/README.txt

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+Date: Wed, 6 Nov 2002 14:05:32 -0600 (CST)
+From: Chris Lattner <sabre@nondot.org>
+To: LLVMdev List <llvmdev@cs.uiuc.edu>
+Subject: What is test/Libraries?
+
+Hey everyone,
+
+There has recently been some confusion over what test/Libraries is and
+what it is used for.  The short answer is: it's used when building GCC,
+not for tests, so you all shouldn't need to use it.
+
+test/Libraries contains the LLVM "system libraries", which are linked to
+programs when the linker is run with the appropriate -l switch (for
+example -lm links in the "math" library).  In general, these libraries are
+just stubbed out libraries, because noone has had the time to do a full
+glibc port to LLVM.
+
+Problems arise because the makefiles have a number of hardcoded paths in
+it that are used to copy files around and install the libraries, which
+cause problems if anyone (except for me) uses them.  I'm sorry a better
+system isn't in place yet for these libraries, but if you just ignore
+them, they won't cause you any harm.  :)
+
+-Chris
+