Update README.md

Update readme in docs. test=doc

paddleaudio/docs/README.md

 # Build docs for PaddleAudio
 
+Execute the following steps in **current directory**.
+
 ## 1. Install
 
-`pip install Sphinx`
-`pip install sphinx_rtd_theme`
+`pip install Sphinx sphinx_rtd_theme`
 
 
 ## 2. Generate API docs
 
-Exclude `paddleaudio.utils`
+Generate API docs from doc string `*py`
 
 `sphinx-apidoc -fMeT -o source ../paddleaudio ../paddleaudio/utils --templatedir source/_templates`
 
@@ -16,3 +17,8 @@ Exclude `paddleaudio.utils`
 ## 3. Build
 
 `sphinx-build source _html`
+
+
+## 4. Preview
+
+Open `_html/index.html` for page preview.