Create README for gRPC Python reflection package

Currently, `warning: sdist: standard file not found` warning is always
printed when creating source distribution for gRPC python reflection
package. This should stop that.

src/python/grpcio_reflection/README.rst

@@ -0,0 +1,10 @@
+gRPC Python Reflection package
+==============================
+
+Reference package for reflection in GRPC Python.
+
+Dependencies
+------------
+
+Depends on the `grpcio` package, available from PyPI via `pip install grpcio`.
+