Add how to develop on README (#4)

Author: harupy

README.md

@@ -1,2 +1,24 @@
 # sphinx-plotly-directive
-A directive for including a Plotly plot in a Sphinx document.
+
+A directive for including a Plotly figure in a Sphinx document.
+
+## Format code
+
+```
+flake8 .
+isort .
+black .
+```
+
+## Run tests
+
+```
+pytest tests
+```
+
+## Build document
+
+```
+cd docs
+make html
+```