added warning for iterating while mutating a sparse matrix (#415)

Author: matbesancon

src/sparsematrix.jl

@@ -294,6 +294,9 @@ column. In conjunction with [`nonzeros`](@ref) and
           # perform sparse wizardry...
        end
     end
+
+!!! warning
+    Adding or removing nonzero elements to the matrix may invalidate the `nzrange`, one should not mutate the matrix while iterating.
 """
 nzrange(S::AbstractSparseMatrixCSC, col::Integer) = getcolptr(S)[col]:(getcolptr(S)[col+1]-1)
 nzrange(S::SparseMatrixCSCView, col::Integer) = nzrange(S.parent, S.indices[2][col])