Add default introductory code example (exercism#516)

Katrina Owen · behrtam · commit f7a5c3bb2d9a · 2017-10-01T10:48:31.000+02:00
Instead of having logic for a fallback in the backend, we're going to
actually add a snippet.
diff --git a/docs/SNIPPET.txt b/docs/SNIPPET.txt
@@ -0,0 +1,10 @@
+prices = {'apple': 0.75, 'egg': 0.50}
+cart = {
+  'apple': 1,
+  'egg': 6
+}
+
+bill = sum(prices[item] * cart[item]
+           for item in cart)
+
+print(f'I have to pay {bill:.2f}')
