add style_melgan readme, test=tts

examples/csmsc/voc4/README.md

 # Style MelGAN with CSMSC
+This example contains code used to train a [Style MelGAN](https://arxiv.org/abs/2011.01557) model with [Chinese Standard Mandarin Speech Copus](https://www.data-baker.com/open_source.html).