Documentation is generally missing

[kalmarek]

And the remaining one (in README) can be in subtle ways not exactly correct:

README says

The MA.operate!!(op::Function, x, args...) redirects to op(x, args...) if x is not mutable or if the result of the operation cannot be stored in x. Otherwise, it redirects to MA.operate!(op, x, args...).

but actually it redirects only to MA.operate(op, x, args...).

As for the future I'd be great if a tutorial page on implementing e.g. MA for Rationals is provided

[odow]

Closing in favor of #293