Add screenshot and update readme instructions

Author: YunkeXiao

apps/dash-datashader/README.md

@@ -5,10 +5,43 @@ Dash abstracts away all of the technologies and protocols required to build an i
 
 Try out the [demo app here](https://dash-gallery.plotly.host/dash-datashader/).
 
+![screenshot](assets/datashader.png)
 
-To learn more check out our [documentation](https://dash.plot.ly).
 
-### Contributing
+## Requirements
+We suggest you to create a separate virtual environment running Python 3 for this app, and install all of the required dependencies there. Run in Terminal/Command Prompt:
+
+```
+git clone https://github.com/plotly/dash-sample-apps
+cd dash-sample-apps/apps/dash-oil-gas-ternary
+python3 -m virtualenv venv
+```
+In UNIX system: 
+
+```
+source venv/bin/activate
+```
+In Windows: 
+
+```
+venv\Scripts\activate
+```
+
+To install all of the required packages to this environment, simply run:
 
+```
+pip install -r requirements.txt
+```
 
-Changes to master will get deployed automatically.
+and all of the required `pip` packages, will be installed, and the app will be able to run.
+
+## How to use the app
+Run this app locally by:
+```
+python app.py
+```
+Open http://127.0.0.1:8050/ in your browser.
+
+Drag a section of the top graph in order to display the zoomed in graph on the bottom.
+
+To learn more check out our [documentation](https://dash.plot.ly).