Document the cases when a subscription should be billed

[zalmanu]

Add the details about the states of the subscriptions that should be billed. It will make it easier to change the code and the tests if the cases are clearly documented.

[presslabs-bot]

This issue will be automatically closed on Tue, 17 Jan 2017 due to the lack of activity (comments or updates). Closed issues that remain inactive for a long period of time might also get automatically locked.

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cc: @calind, @pdobrescu, @vtemian