Add a very minimal README.txt for examples/PrintFunctionNames.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@90197 91177308-0d34-0410-b5e6-96231b3b80d8

examples/PrintFunctionNames/README.txt

+This is a simple example demonstrating how to use clang-cc's facility for
+providing AST consumers using a plugin.
+
+You will probably need to build clang so that it exports all symbols (disable
+TOOL_NO_EXPORT in the tools/clang Makefile).
+
+Once the plugin is built, you can run it using:
+--
+$ clang-cc -load path/to/PrintFunctionNames.so -plugin=print-fns some-input-file.c
+--