Update Portfolio README.md (microsoft#900)

* Update README.md

* Update README.md

Author: you-n-g

examples/portfolio/README.md

@@ -24,6 +24,7 @@ We use China stock market data for our example.
    unzip -d ~/.qlib/qlib_data/cn_data csi300_weight.zip
    rm -f csi300_weight.zip
    ```
+   NOTE:  We don't find any public free resource to get the weight in the benchmark. To run the example, we manually create this weight data.
 
 2. Prepare risk model data: