added funding info

Author: splitbrain

.github/FUNDING.yml

@@ -0,0 +1 @@
+github: splitbrain

composer.json

@@ -20,5 +20,11 @@
     },
     "require-dev": {
         "phpunit/phpunit": "^8.0"
-    }
+    },
+    "funding": [
+        {
+            "url": "https://github.com/sponsors/splitbrain",
+            "type": "github"
+        }
+    ]
 }