Add customer label explanation and change label references to markdown

Add customer label plus change valid labels to markdown format so they render.

The unchanged labels do not have actual/valid labels behind them. We should consider removing them since they're obviously not used.

cc/ @sytses @dzaporozhets @DouweM What do you think about removing the unused labels?

Label names are now rendered

![Screen_Shot_2016-02-25_at_10.59.03_AM](/uploads/4869416c120422b605df8982a46f6d61/Screen_Shot_2016-02-25_at_10.59.03_AM.png)

See merge request !2972