Update README to clarify that cert, profile and entitlements need amending

[jawj]

This tripped me up last night, so I thought it would be worth clarifying here?

[jawj]

Note: I also find that I have to do a rake clean before a rake testflight, or I get this message:

"Invalid IPA: The get-task-allow values in the embedded.mobileprovision and your binary don't match. Are you sure you created the IPA with the same type of certificate as the one you used to compile it?"

I'm not sure whether or not this would be fixed by issue #20.

[miry]

Also we should mention about to use app.entitlements['get-task-allow'] = false in build configuration.

[phoet]

👍