Indicate uncovered properties are optional

[avivmu]

The text-align property isn't covered in this phase therefore adding it is optional
@chrisdavidmills

[avivmu]

@chrisdavidmills any input?

[chrisdavidmills]

I don't really agree with this. text-align is a pretty basic CSS property, and just because it isn't formally covered until the CSS text module, doesn't mean that a learner can't go and look things up on their own. I think a better solution would be to add a note to the assessment article giving a hint that they might need to look something new up for this one.