Add OP funding file to prove ownership.

Signed-off-by: Conor Svensson <conor10@gmail.com>

Author: conor10

funding.json

@@ -0,0 +1,5 @@
+{
+  "opRetro": {
+    "projectId": "0x4dd10ba330a4a21459e2ba72f15fb5da2a63f7492e2b1567e6669c3d47fe6308"
+  }
+}