Update instructions regarding publishing app

[ashmenon]

Just to note, that in the 'Finish setting up your app' section of your README.md, you have the following point:

'Add in-app purchases items from the Developer Console (activate them but do not publish the app)'

But among the common issues you've listed, 'You can no longer test in app purchases with an apk in draft mode.'

I'd suggest altering the initial instructions to point that out, since the instructions on its own give the impression that you don't have to publish it :)