Add a docstring to the constructor.

[tpapp]

No description provided.

[tpapp]

I guess it would make sense to document the constructor with offsets, too, should I do that?

[tpapp]

Now that I revisit this, I am no longer sure what the constructor with indexes does. I thought it specifies the indices that the result will have, but then map(indexoffset, inds) is not the right function --- consider the pre-#67 behavior (note axes):

julia> O1 =OffsetArray(ones(3, 3), -1:1, -2:0)
OffsetArray(::Array{Float64,2}, -1:1, -2:0) with eltype Float64 with indices -1:1×-2:0:
 1.0  1.0  1.0
 1.0  1.0  1.0
 1.0  1.0  1.0

julia> O2 =OffsetArray(O1, -1:1, -2:0)
OffsetArray(OffsetArray(::Array{Float64,2}, -1:1, -2:0), -3:-1, -5:-3) with eltype Float64 with indices -3:-1×-5:-3:
 1.0  1.0  1.0
 1.0  1.0  1.0
 1.0  1.0  1.0

[timholy]

Ooh, you are right. The behavior you achieved in #67 is the one we want to keep, even if it means breaking stuff.

[timholy]

The necessary breaking changes were made in #90 and this was added as 2b77520