Add purpose section to README

Added purpose section explaining the plugin's functionality.

Author: kevinkhill

README.md

@@ -2,6 +2,9 @@
 
 A plugin for Eleventy to showcase your SCAD files.
 
+## Purpose
+Use Eleventy to generate a site to showcase your OpenSCAD models. This plugin adds `.scad` as a template and will use your system's OpenSCAD to render the file into an STL. An additional HTML file with Three.js STL viewer is gererated as well.
+
 ![NPM Version](https://img.shields.io/npm/v/eleventy-plugin-scad)
 
 ## Install into Project