Write a FAQ with examples of how to do common tasks

[jiawen]

... some of which may not be simple.

Also include some gotchas like how cropping (slicing) doesn't change min.

[dsharlet]

I could think of 4 FAQs so far (some are probably not likely to be frequently asked): https://github.com/dsharlet/array/blob/faq/test/faq.cpp

If you have other questions you'd like to see, any ideas are welcome.