website: add instructions for setting up website

website/README.md

 Contributing
 ----
 
+## Setup Environment
+
+Before making any changes, setup your environment with:
+
+```
+make setup
+```
+
+This will setup Pelican, so that you can build and deploy the website.
+You should only have to run the above command once.
+
 ## Making changes
 
 To make changes to the website, keep the following in mind: