Add link to Soban

Fix #2 (closed)

branching-merging/altstadt.md

+# Altstadt
+
+### Soban
+
+Zwingerstr. 21, Heidelberg, BW 69117
+
+https://www.restaurant-soban.de/speisekarte
+
+#### Pro
+
+- good, spicy Korean food
+
+#### Con
+
+- not many vegetarian/vegan options
+- quite small - best to make a reservation!