Add a README explaining the intended content of this directory.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@16053 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/System/Unix/README.txt b/lib/System/Unix/README.txt
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+llvm/lib/System/Unix README
+===========================
+
+This directory provides implementations of the lib/System classes that
+are common to two or more variants of UNIX. For example, the directory 
+structure underneath this directory could look like this:
+
+Unix           - only code that is truly generic to all UNIX platforms
+  Posix        - code that is specific to Posix variants of UNIX
+  SUS          - code that is specific to the Singule Unix Specification 
+  SysV         - code that is specific to System V variants of UNIX
+
+As a rule, only those directories actually needing to be created should be
+created. Also, further subdirectories could be created to reflect versions of
+the various standards. For example, under SUS there could be v1, v2, and v3
+subdirectories to reflect the three major versions of SUS.