Next steps on trigemu documentation

[jcfreeman2]

Right now the documentation is a single sentence in README.md: "Trigger decision emulator for January 2020 readout application tests". This may be enough assuming there are no plans for further use than what's already been done. Otherwise, it's a matter of determining what users would want to know, which I imagine would primarily be a description of the TriggerDecisionEmulator and FakeTimeSyncSource DAQ modules. This in turn may be as simple as cut-and-pasting the documentation at the top of TriggerDecisionEmulator.hpp into README.md (*) and adding a sentence or two about FakeTimeSyncSource

(*) Normally I'd have concerns about straightforwardly cut-and-pasting a comment from a source file into official documentation because they could drift out of sync, but I imagine there won't be much further development of this code in this package