add csharp snippets

snippets/csharp/ReceivePayment.cs

@@ -0,0 +1,19 @@
+using Breez.Sdk;
+
+public class ReceivePaymentSnippets
+{
+    public void ReceivePayment(BlockingBreezServices sdk)
+    {
+        // ANCHOR: receive-payment
+        try
+        {
+            var invoice = sdk.ReceivePayment(
+                new ReceivePaymentRequest(3_000_000, "Invoice for 3000 sats"));
+        }
+        catch (Exception)
+        {
+            // Handle error
+        }
+        // ANCHOR_END: receive-payment
+    }
+}