example: add jsonencode using extension (#3142)

examples/jsonencode/README.md

@@ -0,0 +1,16 @@
+# JSON Encode Extension
+
+`EncodeExtension` is an implementation of entc.Extension that adds a `MarshalJSON`
+method to each generated type `<T>` and inlines the Edges field to the top level JSON.
+
+### Generate Assets
+
+```console
+go generate ./...
+```
+
+### Run Examples
+
+```console
+go test
+```