README guidance on when to use ArrayViews instead of Base.sub(...)

[JohnHBrock]

The README briefly mentions that ArrayViews uses a more efficient representation than Base.sub. Based on the reply @timholy gave to my julia-users post, I wonder if the README should be updated with more detailed and up-to-date guidance on the advantages of ArrayViews. It sounds like julia 0.4 provides much improved array view functionality, and ArrayViews may be approaching deprecation.

[johnmyleswhite]

I'm not the one who should make a final decision, but I'd be in favor of a PR that changes the README and states that ArrayViews are deprecated and should be replaced by SubArrays.

[simonster]

For indexing etc., SubArrays in 0.4 are indeed as fast as ArrayViews. However, constructing ArrayViews is 2-4x faster than constructing SubArrays, and constructing unsafe views is even faster. So it's a tough call.

[timholy]

That's not always true. For example, in this gist after warmup I currently get:

buildcols_view:   0.008320 seconds (300.00 k allocations: 13.733 MB)
buildcols_sub:   0.033081 seconds (1.20 M allocations: 41.199 MB)
buildcols_slice:   0.023696 seconds (900.00 k allocations: 32.044 MB)
buildrows_view:   0.014851 seconds (300.00 k allocations: 18.311 MB)
buildrows_sub:   0.033664 seconds (1.20 M allocations: 45.777 MB)
buildrows_slice:   0.026448 seconds (900.00 k allocations: 32.044 MB)
buildsubset_view:   0.015230 seconds (270.00 k allocations: 16.480 MB)
buildsubset_sub:   0.017131 seconds (270.00 k allocations: 20.600 MB)
buildsubset_slice:   0.016255 seconds (270.00 k allocations: 20.600 MB)

There's essentially no speed difference in the last case.

In any event, I think the best answer is to modify the README so that all the considerations are presented to the user, so s/he can make the choice.