maybe the doc should add some instructions at geting started section

[aohan237]

the doc surely has these content, but as a newbie it will still be confused.
i think should add instructions on getting started section as follows:

1. input.advance_to: it make dd wont accept input changes older than the time.
2. input.flush: advance_to is just a buffer, flush labeled this will happen.
3. worker.step: the above is just configs, and only work.step will make this happen.
4. when exit dd, dd will call worker.step once, make all the work happens. but when you write your loop or infinite graphs, you should call worker.step whenever you need your computation works
5. in a nut shell, in your program, you should call 1-3 sequencetially, otherwise you maybe see hungs.

i think these will make things easier