Adding initial README_smart_holder.rst. (#2936)

* Adding initial README_smart_holder.rst.

* Adding README_smart_holder.rst to MANIFEST.in and test_files.py.

tests/extra_python_package/test_files.py

@@ -92,6 +92,7 @@ sdist_files = {
     "LICENSE",
     "MANIFEST.in",
     "README.rst",
+    "README_smart_holder.rst",
     "PKG-INFO",
 }