add write file example

Author: typpo

examples/write_file.py

@@ -0,0 +1,20 @@
+from quickchart import QuickChart
+
+qc = QuickChart()
+qc.width = 600
+qc.height = 300
+qc.device_pixel_ratio = 2.0
+qc.config = {
+    "type": "bar",
+    "data": {
+        "labels": ["Hello world", "Test"],
+        "datasets": [{
+            "label": "Foo",
+            "data": [1, 2]
+        }]
+    }
+}
+
+qc.to_file('/tmp/mychart.png')
+
+print('Done.')