arrays - introduce ++ syntax

The `arrays` exercise introduces `for` loops, and within that `for` loop the `++` operator is used. However, we haven't introduced the student to the `++` operator. I think it makes sense here to introduce that operator in `introduction.md` file of the `arrays` exercise. The `after.md` document can then mention the different between `x++` and `++x` and introduce the decrement operator (`x--` and `--x`)

An alternative to the above would be to include the `++` and `--` operator in the `after.md` document of the `numbers` exercise or possibly even the `basics` exercise (not my preference).