Add introductory general principles to benchmarking vignette

Along the lines @st-pasha described here : https://github.com/Rdatatable/data.table/pull/2701#issuecomment-375458540

An example of a misleading `microbenchmark()` due to call overhead and small data is sorely needed, such as I alluded to in the same PR just afterwards here:  https://github.com/Rdatatable/data.table/pull/2701#issuecomment-375466111