Document the new GBMatrix constructor #102

julia> GBMatrix([1,2],[2,3],[1,1], nrows=3, ncols=3)
ERROR: MethodError: no method matching GBMatrix(::Vector{Int64}, ::Vector{Int64}, ::Vector{Int64}; nrows=3, ncols=3)
Closest candidates are:
  GBMatrix(::AbstractVector, ::AbstractVector, ::AbstractVector{T}; combine, fill) where T at ~/.julia/packages/SuiteSparseGraphBLAS/hotIr/src/types.jl:431 got unsupported keyword arguments "nrows", "ncols"
  GBMatrix(::AbstractVector, ::AbstractVector, ::AbstractVector{T}, ::Any, ::Any; combine, fill) where T at ~/.julia/packages/SuiteSparseGraphBLAS/hotIr/src/types.jl:427 got unsupported keyword arguments "nrows", "ncols"
  GBMatrix(::AbstractVector, ::AbstractVector, ::T; fill) where T at ~/.julia/packages/SuiteSparseGraphBLAS/hotIr/src/types.jl:462 got unsupported keyword arguments "nrows", "ncols"
  ...
Stacktrace:
 [1] kwerr(::NamedTuple{(:nrows, :ncols), Tuple{Int64, Int64}}, ::Type, ::Vector{Int64}, ::Vector{Int64}, ::Vector{Int64})
   @ Base ./error.jl:165
 [2] top-level scope
   @ REPL[12]:1

julia> GBMatrix([1,2],[2,3],[1,1], 3, 3)
3x3 GraphBLAS int64_t matrix, bitmap by row
  2 entries, memory: 225 bytes
  iso value:   1

    (1,2)   1
    (2,3)   1