Document clarification that en-sv.fwd and sv-en.rev are *output*

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(for a newcomer it can look like they are expected inputs)

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I think it might also be useful to put the whole of eflomal-align --help in the README, which lets people discover what options and features are available and possible without having to first build (or read the source), though at the cost of another place to keep up-to-date if options change.