remove 'Do Not Use in Production' warning from README because that ship has sailed
clean up README.rdoc
clean up whitespace in README.rdoc
add documentation
add Billing-Logic structure to README
define Independent/Single payment strategies
explain independent/single payment strategies' default payment_command_builder_class
remove outdated junk from README; add BaseStrategy info
improve README
improve README indentation
remove obsolete stuff from README
improve README
improve CurrentStateMixin documentation
improve README
add yard as dev dependency
improve BillingCycle documentation
add Yard docs; fix: ProfileProductOwnership -> BillingEngine::Client::Product

Conflicts:
	lib/billing_logic/billing_cycle.rb
improve README
add documentation; refactor confusing method; make BaseStrategy an abstract class
[Completes #52070441] replace deprecated 'source :rubygems'