Add Quickstart

Quickstart.md

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+To add PyCanvas into your application with skulpt, just add the following to the top of your builtInRead() function:
+```javascript
+if (x === 'screen' || x === './screen.js') {
+    const url = new URL('screen.js', window.location.href).href;
+    const xhr = new XMLHttpRequest();
+    xhr.open("GET", url, false);
+    xhr.send();
+    if (xhr.status === 200) return xhr.responseText;
+    else throw "Could not load screen.js";
+}
+```
+Now set these variables when you run your code:
+```javascript
+const canvas = document.getElementById('canvas');
+const container = document.getElementById('preview-container');
+if (canvas && container) {
+    const w = Math.floor(container.clientWidth);
+    const h = Math.floor(container.clientHeight);
+    canvas.width = w;
+    canvas.height = h;
+}
+```
+
+And create a canvas with the id `canvas` inside a div with the id `preview-container`.
+
+---
+
+To create a basic application, define your functions and pass them to s.start() in a dictionary:
+
+```python
+import screen as s
+
+def draw():
+    s.cls(0)
+    s.text(s.CENTERX, s.CENTERY, "Hello World", 7)
+
+s.start({
+    "loop": {
+        "draw": draw
+    }
+})
+```