Update README (#7)

Author: harupy

README.md

@@ -1,22 +1,44 @@
 # sphinx-plotly-directive
 
+![CI](https://github.com/harupy/sphinx-plotly-directive/workflows/CI/badge.svg)
+[![Documentation Status](https://readthedocs.org/projects/sphinx-plotly-directive/badge/?version=latest)](https://sphinx-plotly-directive.readthedocs.io/en/latest/?badge=latest)
+
 A directive for including a Plotly figure in a Sphinx document.
 
-## Format code
+## Install
+
+```
+pip install git+https://github.com/harupy/sphinx-plotly-directive.git
+```
+
+## How to use
+
+```
+.. plotly::
+
+   import plotly.express as px
+   px.scatter(x=[0, 1, 2, 3, 4], y=[0, 1, 4, 9, 16])
+```
+
+The last line of a code block must end with an expression that evaluates to a plotly figure.
+
+## Development
+
+### Format code
 
 ```
 flake8 .
 isort .
 black .
 ```
 
-## Run tests
+### Run tests
 
 ```
 pytest tests
 ```
 
-## Build document
+### Build document
 
 ```
 cd docs