Following the README.md, the first time to write the doc '_design/mailbox' will get an error

Following the document README.md to create a filter, when I type `wsk action invoke /_/myCloudant/write -p dbname testdb -p overwrite true -P design_doc.json -r` and return, I will get an error:
```
"Error: missing\n at Request._callback (/nodejsAction/node_modules/cloudant-nano/lib/nano.js:247:15)\n at Request.self.callback (/nodejsAction/node_modules/request/request.js:186:22)\n at emitTwo (events.js:106:13)\n at Request.emit (events.js:191:7)\n at Request. (/nodejsAction/node_modules/request/request.js:1081:10)\n at emitOne (events.js:96:13)\n at Request.emit (events.js:188:7)\n at IncomingMessage. (/nodejsAction/node_modules/request/request.js:1001:12)\n at IncomingMessage.g (events.js:291:16)",
```

I think here we should remove `-p overwrite true` since it's the first time to create the doc '_design/mailbox'. README.md should be updated.