Clarify README about using gcc/clang -E as cpp.

In relation to #32.
diff --git a/README.rst b/README.rst
index 5a4e637..4bb48dd 100644
--- a/README.rst
+++ b/README.rst
@@ -130,6 +130,9 @@
 <http://www.cs.princeton.edu/software/lcc/>`_, and is provided under LCC's
 license terms.
 
+Note also that you can use ``gcc -E`` or ``clang -E`` instead of ``cpp``. See
+the ``using_gcc_E_libc.py`` example for more details.
+
 What about the standard C library headers?
 ------------------------------------------