[New Tip] Design tip no1 - Providing sufficient contrast

[dboudreau]

Reference: http://w3c.github.io/wai-quick-start/designing.html#provide-sufficient-contrast-between-foreground-and-background

The description of this quick tip ends with a sentence that says: "Contrast ratio is a short version of the more technically correct term luminance contrast ratio". Having taken part in the "Easy Checks" discussions a few years ago, I understand where that last sentence about luminance is coming from, but this is way too subtle for anyone who's new to accessibility. In order to remain true to our goal of addressing readers in layman's terms, I suggest we remove the last sentence entirely. I don't think it brings any real value to someone who would be new to accessibility.