add csharp snippets

snippets/csharp/BuyBtc.cs

@@ -0,0 +1,19 @@
+using Breez.Sdk;
+
+public class BuyBtcSnippets
+{
+    public void BuyBtc(BlockingBreezServices sdk)
+    {
+        // ANCHOR: buy-btc
+        try
+        {
+            var buyBitcoinResponse = sdk.BuyBitcoin(
+                new BuyBitcoinRequest(BuyBitcoinProvider.MOONPAY));
+        }
+        catch (Exception)
+        {
+            // Handle error
+        }
+        // ANCHOR_END: buy-btc
+    }
+}