added two sourdough bread recepies (#446)

src/sourdough-potato-bread.md

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+# Potato sourdough bread
+
+![Potato sourdough bread](pix/sourdough-potato-bread.webp)
+
+- ⏲️ Prep time: 15 min
+- 🍳 Cook time: 45 min
+
+## Ingredients
+
+- 150g boiled potatoes in there skin
+- 350g wheat flour
+- 50g whole rye flour
+- 11g salt
+- 150g water
+- 250g sourdough
+
+## Directions
+
+1. Boil the potatoes
+2. Peel of the skin of the potatoes (let them cool down a bit)
+3. Stir all ingredients and knead them well.
+4. Let the dough rest for 5 hours in warm (25-30°C) conditions.
+5. Preheat the oven to 250°C.
+6. Bake for up to 10 minute with steam.
+7. Release the steam and turn the heat down to 220°C.
+8. Bake for another 35 minutes
+
+## Contribution
+
+SirStart
+
+;tags: sourdough bread potato